C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7

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Structural phase transitions of C60

under high-pressure and high-temperature

http://dx.doi.org/10.1016/j.carbon.2014.10.0830008-6223/� 2014 Elsevier Ltd. All rights reserved.

* Corresponding author.E-mail address: michelle.alvarezmurga@gmail.com (M. Alvarez-Murga).

M. Alvarez-Murga *, J.L. Hodeau

Institut Neel, CNRS/UJF UPR2940, 25 rue des Martyrs BP 166, 38042 Grenoble Cedex 9, France

A R T I C L E I N F O A B S T R A C T

Article history:

Received 31 July 2014

Accepted 24 October 2014

Available online 8 November 2014

Structural phase transitions of C60 under high-pressure and high-temperature are pre-

sented in a concise overview extracted from updated literature. Primarily focused on poly-

merization of C60: ordered-1D, 2D, 3D polymers- and disordered-graphite-like and

diamond-like-phases as well as spatially constrained 1D fullerenes-encapsulated fuller-

enes, i.e. peapods. Special accent is given to X-ray diffraction data addressing complexity

and limitations on data analysis and interpretation for low atomic number, multiphase,

textured and disordered samples. Cutting-edge experimental work, description of novel

X-ray based experimental methods as well as outline for future directions in this research

field are presented.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

C60 was discovered three decades ago [1,2] and since then

numerous research works have been devoted to understand

the structural phase transitions and reaction diagram of this

carbon allotrope [3–6]. Research on C60 phase transformations

reached its apex during the 90’s. It allowed the discovery of

ordered polymeric structures and the outlining of a tentative

reaction diagram built-up mainly from ex-situ results [4].

Despite the importance of this system from a fundamental

point of view, its applications in the fullerene-based chemis-

try field and as potential ‘‘ultra-hard’’ materials, in recent

years the attention of researchers has focused on resolving

the structural diagram of materials with more promising

properties, such as carbon nanotubes or peapods [7]. The lat-

ter can be prepared by relatively accessible techniques as they

can be dispensed from the use of high-pressure methods,

which is not the case for some of the C60 derived polymorphs

[8]. Recent advances in characterization techniques [9] open a

way to analyze complex multiphase and disordered materials

making possible and interesting to reexamine the reaction

diagram of C60. The clear vision of this research area and

novel analysis methods applied on C60 might be of benefit

for scientists working with fullerenes and other carbon

allotropes.

This paper aims to present a clear, concise and updated

overview of the vast literature covering structural phase tran-

sitions of C60. Primarily focused on polymerization of C60:

ordered (1D, 2D and 3D polymers) and disordered (graphite-

like and diamond-like) phases as well as spatially constrained

1D fullerenes (encapsulated fullerenes, i.e. peapods). This

paper will not attempt to include the case of doped C60 poly-

mers, as their proper treatment would demand a deeper dis-

cussion an analysis of physical properties of the doped

phases. Therefore, the text refers to undoped bulk powder

polymers unless another mention is written. Emphasis is

given in results obtained from structural probes such as X-ray

diffraction (XRD) and Raman spectroscopy. Cutting-edge

382 C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7

methods for upgraded analysis are described. Accessible

regions of the P–T diagram which remain unexplored are

presented.

The content is organized as follows: Section 2 introduce

the historical context and scientific interest of high-pressure

research on C60. Section 3 describe the preparation of C120

dimers upon radiation, solid-state reactions or high-pressure

treatment of C60 monomer. Section 4 present the molecular

orientations allowing formation of different types of intermo-

lecular bonds between bulk or confined C60 molecules.

Emphasis is given to orientational ordering and structural

phase transitions occurring at low temperature. Section 5

summarize the 1D orthorhombic structures formed in the

low pressure range P < 2 GPa and in the temperature range

between 500 and 700 K. and describes the polymerization

and coalescence of 1D arrays of confined C60 molecules, the

so called C60 peapods. Section 6 discuss on the 2D tetragonal

and rhombohedral polymers (P � 2–8 GPa) and focuses on the

structural organization and transformation mechanism into

nanostructured graphite-like carbon derived at high tempera-

tures. Section 7 expose the current knowledge on 3D ordered

polymers (P < 8 GPa) and their corresponding disordered dia-

mond-like carbon phases obtained at high temperatures. Sec-

tion 8 present recent advances and cutting-edge methods for

XRD analysis of heterogeneous systems, applied on C60 and

suggest accessible areas of the P, T diagram that merit further

exploration using basic or advanced methods.

2. Historical context and scientific interest forhigh-pressure research on C60

2.1. Why researchers squeezed C60 in the early 90’s?

Back in 1985, Kroto and collaborators [1] identified a new car-

bon allotrope C60 (Fig. 1), so called Buckminsterfullerene or

fullerene. Early after their discovery, large pure quantities of

C60 molecules became available for research [2]. Since then,

Fig. 1 – C60 molecule has a soccer-ball shape of �0.709 nm in diam

pentagons (6 fivefold axes). The double bonds between two hexa

bonds between hexagons and pentagons (�1.455 A) [13]. (b) Th

molecules, imposing some degree of angular strain in the structu

the sp2 orbitals. The C–C–C bond angles are �108� for planar ca

graphene. (c) Schematic representation of molecules positioned

molecular solid (a = 14.17 A) at room pressure and temperature.

the exploration of chemistry and physics of fullerene-based

materials has been very intense and so far C60 remains the

most studied member of the fullerene family. In the early

90’s, theoretical studies predicted a very large bulk modulus

for C60 molecules (B0 � 700 GPa) and it was suggested that a

C60 crystal could be stiffer than diamond (B0 � 441 GPa) [10–

12]. Since then, a number of compression experiments have

been conducted in order to determine the bulk modulus and

phase transitions of C60 molecular solid.

2.2. What happened when C60 was squeezed and heated?

Since the very first compression experiments carried on C60

[10–12], it was demonstrated that mixtures of polymeric

structures are formed. Let us present the polymerization of

C60 in a nut-shell. C60 molecules are joined by weak Van der

Waals interactions at room pressure (P) and temperature (T)

and close-packed as a molecular solid with an average

face-centered-cubic (f.c.c.) structure. At these conditions

C60 molecules can ‘‘freely’’ rotate around themselves. At low

temperature rotation is locked to discrete positions giving rise

to a lower symmetry solid. Under high-pressure and high-

temperature treatment (HP–HT) the interfullerene distance

gets shortened and the orientational disorder of the C60

molecules (Section 4) favors the appearance of new intermo-

lecular bonds. Typically, two C60 molecules get linked through

four-membered rings generated by [2 + 2] cycloaddition

reactions in the so called dimerization process which can also

be attained by other means (Section 3). This leads to the for-

mation of (C60)2 dimers (P � 1 GPa) and C60 polymers: 1D-lin-

ear (P < 2 GPa), 2D-layered (P = 2–8 GPa) and 3D-network

(P > 8 GPa) structures (Sections 5–7). In the 1D-polymers, the

C60 molecules are linked into polymeric chains whereas in

the 2D-polymers, square (tetragonal) or hexagonal (rhombo-

hedral) polymerized layers are formed. Mixtures of 2D-rhom-

bohedral and 2D-tetragonal structures are usually observed

below 4–5 GPa and single phased 2D-rhombohedral structures

eter composed of (a) 20 hexagons (10 threefold axes) and 12

gons (15 twofold axes) are shorter (�1.391 A) than the single

e presence of pentagons induces the curvature of the C60

re and increasing the reactivity through pyramidalization of

rbon pentagon, that otherwise will be �120�, as in planar

in the f.c.c. lattice. C60 molecules are close-packed in a f.c.c.

C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7 383

are obtained in the 4–8 GPa range. Disordered graphite-like

carbon phases are produced from the HT treatment

(�1000 K) of the 2D-polymers. The 3D-polymers can be both

crystalline and/or disordered sp2–sp3 carbon networks joined

by covalent bonds [14]. Disordered diamond-like carbon

phases are produced from the HT treatment (�1000 K) of the

3D-polymers. The measured hardness of these materials is

comparable to that of cubic boron nitride (cBN) [15].

2.3. Why researchers continue to squeeze C60 in 2014?

Despite the significant amount of work carried on the struc-

tural phase transitions of C60, many regions of its P–T reaction

diagram continue to be unclear and open to discussion

(Fig. 2). We summarize herein several factors limiting the

interpretation of the structural data that has been collected

over the years. So far most X-ray diffraction or Raman spectra

have been measured on quenched samples, after recovering

them to ambient conditions. It is well known that during

the quenching process reversible-phase transitions, amorph-

ization or texturing could occur. The preparation path

through the reaction diagram (T–P vs. P–T) [16,17] and the

treatment times (short (min) vs. long (hours), slow transfor-

mation kinetics) [18,19] can greatly influence the complete-

ness of the transformations and the degree of order of the

polymerized structures. Additionally, the hydrostaticity of

the sample environment and the type of compression can

play an important role on bonding direction and ordering dur-

ing C60 polymerization [20]. Furthermore, at highest pressure

and/or temperature, C60 cages ‘‘collapse’’, giving rise to new

‘‘disordered’’ or amorphous-like products whose characteris-

tics are still under debate. Moreover, the low scattering power

of carbon atoms and the similar densities of the different C60

polymorphs call for the development of high-resolution

structural probes capable to follow in situ the structural evolu-

tion during polymerization. Nevertheless, despite all the

experimental difficulties mentioned before, well character-

Fig. 2 – Simplified P–T diagram of C60. Summary of the

structural knowledge obtained from quenched samples.

Molecular motifs are shown for the C60 monomer, dimers,

1D and 2D polymers. Original figure can be found in

Sundqvist B. Phys Stat Sol (B) 2001; 223:469–477. (A color

version of this figure can be viewed online.)

ized ex-situ structures have been identified in different

regions of the P–T diagram (Fig. 2).

3. Dimerization (C60)2

C60 reactivity is that of an electronegative molecule, accessi-

ble to chemical reductions, radical or nucleophilic additions

and various [2 + n]-cycloaddition reactions with n = 1, 2, 3 or

4 the most common [21]. Fullerene dimers can be prepared

by [2 + 2] cycloaddition reaction through different methods

such as photoreaction [22], solid-state mechanochemical

reaction [23,24] or thermal compression above �1 GPa [25].

These preparation methods yield C60 dimers with dumb-bell

D2h symmetry [22–25]. Their structure consists of two C60

units covalently linked through intermolecular four-mem-

bered carbon rings which are generated by the breaking of

double-bonds and reforming of links between neighboring

C60 molecules (Fig. 3). Doped fullerene dimers such as KC60

and RbC60 show a different structure, where charged (C60)2�2

dianions are covalently linked by single C–C interfullerene

bonds [26].

3.1. Photopolymerization

Radiation such as UV or visible light can be used to produce

radicals from the starting C60 monomer and achieve photo-

transformation [22]. However, the efficient light absorbing

properties of C60 and the limited penetration depth of the

light (<1 lm) constraint photoreaction volumes to very thin

films (<10 lm) or surfaces. The dimeric product is insoluble

in toluene but dissociates back to monomeric C60 when

heated above 400 K. Photoinduced polymerization occurs

above 260 K. At this temperature C60 undergoes a phase tran-

sition from a low-temperature simple-cubic (s.c.) orientation-

ally ordered structure (space group Pa3) to a high-temperature

orientationally disordered one (space group Fm-3m) [27]. The

formation of four-membered carbon rings between adjacent

molecules requires free rotation of the molecules for align-

ment of the double bonds. The structure of the phototrans-

formed material depends on the transformation

temperature: at 320 K it contains mainly (C60)n clusters

whereas above 360 K dimers are obtained. Formation of

longer linear polymers is not observed [28]. Photopolymeriza-

tion can also induce branching between the chains which can

be characterized by a Raman mode near 1454 cm�1. These

units are also highly disordered [29]. The phototransforma-

tion process decreases the symmetry of the molecule produc-

ing changes in the IR and Raman spectra. These features are

used as signature to distinguish between monomeric and

dimeric products (Table 1). The Ag(2) pentagonal pinch mode

(1469 cm�1) dramatically loses intensity and new features

become observable at its low energy side (Fig. 4). Ag(1) and

Hg(1) modes show line splitting and new low frequency

modes assigned to inter-ball stretching appear [30].

3.2. Solid-state mechanochemical reaction

Bulk synthesis of C120 dimers can be achieved by high-speed

vibration milling of C60 in the presence of different reagents

Fig. 3 – The molecular structure of C120 determined by X-ray crystallography. Selected bond lengths and angles are as follows:

C1–C1*, 1.575(7); C1–C2, 1.581(7); C2–C3, 1.530(8); C3–C4, 1.374(7); C4–C5, 1.468(8); C5–C6, 1.358(9); C6–C1, 1.528(7); C6–C7,

1.445(8); C7–C8, 1.457(9); C2–C1–C6, 115.4(5); C2–C1–C9, 115.2(4); C6–C1–C9, 100.7(4); C2–C1–C1*, 90.3(4); C1–C2–C2*, 89.7(4).

Original figure can be found in Wang et al. Nature 1997; 387: 583–6 [23].

Table 1 – Summary of Raman modes used to distinguish between monomeric and dimeric C60.

Type of sample Ag(2) (cm�1) Ag(1) (cm�1) Hg(1) (cm�1) Low frequency modes (cm�1)

Monomeric C60 at 298 K 1469 496 267 –Phototransformed C60 at 300 K 1458 Multiple Strong 117

1452 Splitting Line splittingPhototransformed C60 at 380 K 1462 490 Line splitting 96

117 (weak)

Fig. 4 – Raman signature of photopolymerized C60 in the range of the (a) Hg(1) mode, (b) of the Ag (1) mode and of the (c) Ag (2)

mode. The transformation temperature is indicated. The spectrum of C60 at 450 K is shown for comparison. The spectra

shown are fully transformed samples with a stable Raman response. Reprinted with permission from Burger et al. Z Phys B

1996; 101(2): 227–33 [28]. Copyright (1996), with kind permission from Springer Science and Business Media.

384 C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7

such as potassium (K) salts (e.g. KCN, KOAc, K2CO3, KOH and

CH3CO2K) [23], small amounts of metals (e.g. Li, Na, K, Mg, Al,

and Zn) or organic amines (e.g. 4-(dimethylamino)- and 4-

aminopyridine) [31]. The nucleophilic and radical versions of

the possible reaction mechanism are presented in Fig. 5.

According to Komatsu et al. [31] the radical path is the most

probable as the chain process can be triggered by small

amounts of reducing metals. The equilibrated reaction yield

is about 30% regardless the reagent used [23,31]. Except for

the reaction with 4-aminopyridine where the yield for C120

is around 45% and 4% yield of fullerene trimer C180 is

obtained. The dimer reverts back to C60 when exposed to

Fig. 5 – Reaction mechanisms for synthesis of C120. The C60 radical anion is formed either by direct one-electron reduction of

C60 or by one electron transfer from the cyanated C60 anion, C60(CN)�. Under the present solid-state reaction conditions the

radical anion reacts with neutral C60 to give the C120 radical anion, which can transfer one electron to provide another C60

radical anion inducing the chain reaction. Through these reactions, an equilibrium of ratio 7:3 is attained between C60 and

C120. Reprinted with permission from Komatsu et al. Carbon 2000; 38: 1529–34 [24]. Copyright (2000), with permission from

Elsevier.

C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7 385

heat, visible light, grinding or electrochemical reduction

[31,32].

3.3. High-pressure treatment

Fullerene intermolecular distances decrease under high-pres-

sure whilst orientational disorder increase under high-tem-

perature promoting favorable conditions for [2 + 2]

cycloaddition reactions. Bulk powder dimers can be prepared

in a broad pressure range above �1 GPa up to at least 10 GPa,

at room or high temperature [25]. Their formation is evi-

denced by the appearance of previously icosahedral symme-

try-forbidden bands at 796 and 478 cm�1 in the IR spectra.

C120 is not a stable phase, but an intermediate state before

the formation of 1D-linear or 2D-layered polymers. Energy-

calculations performed for different structures of dimers,

based on the f.c.c. lattice packing, show that the stability field

of the theoretical structures is pressure dependant. One of the

hypothetical structures could be geometrically favorable for

the formation of an infinite polymer [33]. Experimentally it

has been found that he polymerization rate depends on the

reaction conditions, P and T, but also on the rotational and

orientational states of the molecules. The polymerization

yield depends on the choice of the reaction path either by iso-

baric heating or isothermal pressurization [34]. A dimeriza-

tion kinetics study performed later shows that activation

energies are slightly different for the f.c.c. packing (Eafcc =

�121 kJ mol�1) and the s.c. packing (Easc = �137 kJ mol�1)

[35]. This might be explained by the fraction of pentagonal

(P) and hexagonal (H) orientational states (see Section 4 for

details) present in each phase. Due to orientational disorder,

the f.c.c. symmetry shows a larger fraction of H-orientations

where double-bonds are well-disposed for [2 + 2] cycloaddi-

tion reactions. In the s.c. phase, the fraction of the P-oriented

molecules is more important, however, higher pressures favor

molecular transition from P-oriented to H-oriented. The for-

mation of the first dimers under pressure supplies some ori-

entational disorder to the s.c. lattice. The reaction is then

catalyzed and the process becomes similar to that of the

f.c.c. phase. In a recent publication semi-quantitative reaction

maps are given for the formation of dimers and chains [36].

So far, identification of dimers among other polymer prod-

ucts remains a difficult task due to their randomly disordered

nature, the presence of variable fraction of monomers, tri-

mers or polymers which are not clearly dissociated by

changes in the IR or Raman spectra. Dimer single crystals

can be synthesized in the 1–6 GPa range. X-ray diffraction

show they were formed of some disordered dimers with

minor fractions of C60 monomer, 1D, 2D polymers and proba-

bly some trimers [37]. The diffuse scattering intensities indi-

cated that dimers were both positionally and orientationally

disordered within an average cubic lattice. Distinguishing

C60 monomer from dimers in powder X-ray diffraction (XRD)

patterns is not so obvious. Essentially diffraction peaks

broaden due to disorder and shift to larger angles as conse-

quence of the lattice parameters reduction, but the lattice

remains cubic in average. The reduction of the lattice con-

stant might not be isotropic as a consequence of the stress

applied along preferential axes as often imposed by the

high-pressure devices. The need for more in situ studies at

low pressures has been addressed since long time as this is

crucial to analyze the next stage of polymerization which

Fig. 6 – The P and H orientations. The black lines represent

<111>cubic directions, the plane of the drawing is normal to

one of them. The rods indicate <110>cubic directions, at

35.26� from <111>cubic. To build these configurations: (i) a

three-folded axis coincide with one [111]cubic direction, (ii)

six double bonds, normal to (i) straddle cubic <110>cubic

directions and (iii) (a) three fivefold axes (normal to

pentagons, P) or (b and c) three threefold axes (normal to

hexagons, H) lie close to cubic <110>cubic directions.

Reprinted with permission from Copley, David, Neumann.

Neutron News 1993; 4(4): 20–8. Reprinted by permission of

Taylor & Francis Ltd, http://www.tandfonline.com.

386 C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7

consist in the formation of ordered 1D-linear chains. There is

a lack of information concerning the conversion yield as a

function of reaction time and very little is known about the

structural characteristics of the dimers as a function of

pressure.

4. Orientation of C60 molecules

4.1. Orientational phenomena

The high symmetry of the C60 molecule induces three-dimen-

sional orientation conformations that influence the physical

properties and compressibility of the monomer as well as

the polymerization process. A compelling critical review of

the orientational properties of bulk C60 has been presented

by Moret in 2005 [6]. Here we summarize the most important

aspects of molecular orientation occurring in bulk and con-

fined C60 which are crucial to understand the formation of

intermolecular bonds and higher order polymerization

processes.

Essentially, under thermal agitation C60 molecules rotate

around their centers, producing rapid reorientations. At room

temperature, the average structure is f.c.c. (Fm-3m,

a = 14.17 A) with a high degree of orientational disorder [38].

Experimentally, Bragg reflection intensities (from neutron or

X-rays) have been used to extract the average single-molecule

orientation probability density f(x), with x the Euler angle of

their molecular orientation [39–43]. This experimental orien-

tation probability has been approximated using a simple

homogeneous spherical-shell model [44,45], and also a model

of delocalized molecular charge densities [46–49]. The diffuse

scattering intensities and their modulations in reciprocal

space have been used to evaluate intermolecular correlations

[46–48]. These studies indicate that the double bonds (DB) of

C60 molecules have a tendency to face either pentagons (P–

DB) or hexagons (H–DB) of the neighboring molecules.

For a C60 molecule, we can define x as the angle of rotation

around a threefold axis parallel to a <111> cubic axis. Using

this reference, two preferred orientations have been identified

(Fig. 6) and denoted P for pentagon (x = 98�) and H for hexagon

(x = 38�) [40,49]. The transformation of P to H involves a 60�rotation around the <111> cubic directions (Fig. 6a and b) or

a �42� rotation about the <110> cubic directions (Fig. 6a and

c). These reorientations are not equivalent; while a large 60�rotation involves breaking of all P–DB and H–DB bonds the

short 42� rotation preserves some contacts. According to cal-

culations of rotational hindrance potential, the short rotation

is more likely to occur, this means that interaction potential

between neighboring molecules constrains molecular reori-

entation [43,50].

The behavior of C60 monomer at low pressures and low

temperatures is well described (Fig. 7). Solid C60 crystallizes

into a f.c.c. structure at room temperature, where molecules

perform nearly free rotations (300–260 K). Two important

phase transitions take place at low temperatures: appearance

of an orientationally orderer low-symmetry s.c. phase (260–

1 There is a large spread in the experimental values of dTo/dP mainlygases such as He into C60.

100 K) and transition into a glassy state where molecular ori-

entations are ‘‘frozen’’ (Tg � 85 K) [38].

4.2. Orientationally order low-temperature phase

At temperatures below To � 260 K a first-order phase

transition occurs (Fig. 7) lowering the symmetry from f.c.c.

(Fm-3m) to s.c. (Pa-3) [27,40]. In this configuration all the mol-

ecules have their threefold axis oriented along one of the four

<111>cubic directions. In the orientationally ordered s.c.

phase, P–DB and H–DB near neighbors configurations coexist,

but the fraction of molecules with P-orientation is larger. This

is also observed for the glassy state. However, the fraction of

H-oriented molecules increases with pressure, leading to a

50P/50H distribution above �0.2 GPa [41]. The pressure depen-

dent shift from P to H orientation is due to the difference in

molecular volume between the two states [51,52]. The pres-

sure dependent orientational ordering transition temperature

has an estimated1 slope of dTo/dP � 165 K GPa�1. Therefore, at

room temperature, the f.c.c. to s.c. transition takes place at

about 224 MPa [53–55].

due to artefacts induced by intercalation of pressure-transmitting

Fig. 7 – Low temperature P–T diagram of C60. The solid lines

indicate the f.c.c. to s.c. transition. The dotted line indicate

the glass transition. The almost vertical broken line denotes

the equilibrium between the P and H orientations and the

shaded area is the region where a transition into an H-

oriented phase should occur. Reprinted with permission

from Sundqvist B. Advances in Physics 1999; 48(1): 1–134 [4].

Reprinted by permission of Taylor & Francis Ltd, http://

www.tandfonline.com.

Fig. 8 – Orientation of encapsulated fullerenes inside carbon

nanotubes. The circle represents the tube wall. (a) pentagon

(P), (b) double-bond (DB), (c) hexagon (H), and (d) vertex (V)

orientations. Original figure can be found in Zhen et al. Chin

Phys B 2013; 22(7): 076101 [66].

C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7 387

4.3. Orientationally disordered glass

Below Tg � 85 K, the fraction of P-oriented molecules satu-

rates and a glassy state where rotations are ‘‘frozen’’ appears

[40]. In the glassy state, P/H reorientations are frustrated as

the thermal energy is not enough to overcome the energy

gap between the two orientations [56]. This glass transition

temperature also increases with pressure, with a slope of

dTg/dP � 62 K GPa�1 as determined from thermal conductivity

measurements [57].

4.4. Orientation of confined C60

Investigating the rotational states of confined molecules is

interesting from a theoretical point of view (e.g. how C60 mole-

cules rotate inside carbon nanotubes at room temperature is

still an open question) and also to design new materials with

tuned electronic and mechanical properties [58,59]. For exam-

ple, C60 molecules inside single-walled carbon nanotubes

(SWCNT), so called peapods (C60@SWCNT), are potential candi-

dates to build nano-scaled memory devices [60], superconduc-

tors [61–64] or CNT-based organic electronics [65]. The high

symmetry of C60 molecules and their weak intermolecular

interactions provide an ideal model to understand confine-

ment effects in one-dimensional constrained nanostructure

systems. The behavior of C60@SWCNT in which each C60 mole-

cule has only 2 nearest neighbors is expected to be different

from that of bulk C60 which has 12 neighbors per molecule.

(e.g. Van der Waals interactions and Coulomb attraction).

Theoretical studies performed on peapods show three

lowest energy states, corresponding to orientations of the

molecules inside the SWCNT: pentagonal (P), hexagonal (H)

and double-bond (DB) orientations (Fig. 8) and more recently

a vertex (V) orientation [66]. These preferable orientations

are independent of the tube chirality, but depend on the tube

diameter [67–69]. Theoretical studies have been extended for

confined C70 and C80 [69–72]. Inelastic neutron scattering

shows that ‘‘quasi-free’’ rotational diffusion of C60 molecules

in peapods is observed at room temperature [73] and persist

until a lower temperature (100 K 6 Tconfined 6 200 K) compared

to bulk C60 (100 K 6 Tbulk 6 260 K) [74].

Recent near-infrared Raman studies performed on pres-

surized samples show that C60 molecules inside SWNTs exhi-

bit an unusual type of ratcheted rotation with a preferred

‘‘hexagonal orientation’’ due to the interaction between C60

and SWNT walls which is enhanced during dimerization.

[75] While molecules in pentagon-to-pentagon orientation

cannot polymerize, the hexagon-to-hexagon orientation is

advantageous for the formation of covalent intermolecular

bonds and low-temperature polymer. C NMR spectroscopy

shows that encapsulated C60 molecules remain monomeric,

preserve high rotational mobility down to very low tempera-

ture (T � 30 K) without any orientational phase transition.

This is related to the fact that a 1D crystal does not undergo

a phase transition at finite T, as long range-order is not

obtained [76], as previously reported for C70 [77,78]. However,

other C NMR spectroscopy results performed on highly-puri-

fied samples evidence that encapsulated C60 undergoes a

phase transition from continuous rotational diffusion (orien-

tational disorder) to uniaxial rotations at To � 100 K (e.g.

for unconfined C60 orientational order is obtained below

To � 260 K [79]) and finally freeze out at Tg � 25 K (for uncon-

fined C60 Tg � 85 K [80]) [81]. Some discrepancies between

these data sets around 100 K could be also explained by the

388 C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7

difference on SWNT samples (e.g. diameter, chirality). A recent

theoretical study investigates the preferred orientation of a

confined C60 molecule when the nearest neighbor (two molecules

model C60a–C60b) was fixed at the -P, -DB, and -H orientations

respectively [66]. Three preferred orientation of the C60a mol-

ecule are identified depending on the SWNT diameter: penta-

gon (dSWCNT < 1.31 nm), hexagon (dSWCNT > 1.36 nm), and for

(1.31 < dSWCNT < 1.36) transition from -P to -H (with C60b in -P

or -DB orientation) and -V (with C60b in -H orientation). The

same team of researchers [82] has developed a novel method

based on the Van der Waals energy map to investigate

preferred orientation of confined molecules inside boron

nitride nanotubes (C60@SWBNNT) and carbon nanotubes

(C60@SWCNT) related to the symmetry axis rather than the

classic -P, -H, or -DB orientations [67–69]. Analytical expres-

sions to calculate the Van der Waals interaction energy for

the various orientation configurations that can be adapted

by spheroidal molecules (lying, standing, tilted, with off-

and on-axis variants) have been derived [60]. A similar treat-

ment is applied for nonlinear zigzag and spiral configurations

of fullerene chains inside carbon nanotubes [83].

5. 1D orthorhombic polymers

Disordered branched chains of C60 are obtained by photopoly-

merization (Section 3) [34] whereas 1D ordered structures are

formed by high-pressure-high-temperature treatment of

Table 2 – Crystallographic data for the 1D orthorhombic polymecorresponding theoretical calculations.

Report on Phase Space group a (A) b (A)

Experimental dataPowder O Immm 9.26 9.88Powder O 0 Immm 9.09 9.83Single-Crystal O 0 Pmnn 9.14 9.90

Theoretical calculationsTheory O Pnnm – –Theory O 0 Pmnn – –

Fig. 9 – 1D chain orientations. The chain orientation is defined b

ring here represented as shaded bars) and the <001>cubic direct

<110>cubic directions. The broken lines --- represent the 2D lay

connecting chains through supplementary four-membered rings

2000; 544: 81. Copyright (2005), AIP publishing LLC.

unconfined (Section 5.1) or spatially constrained C60 mole-

cules (Section 5.2). Two main mechanisms can be distin-

guished for molecular connection in this system:

polymerization (Sections 5.1 and 5.2.1) and coalescence (Sec-

tion 5.2.2).

5.1. Polymerization of unconfined C60

For unconfined pure C60, formation of ordered 1D linear

chains of C60 molecules linked by four-membered carbon

rings occurs in the low pressure range (P < 2 GPa) and in the

temperature range between 500 and 700 K (Table 2). The ini-

tial structure is cubic in average but contracted along the

(110) direction resulting in an orthorhombic lattice of poly-

merized chains of C60 molecules (Fig. 9). Several experimental

works covering the low pressure range have reported the syn-

thesis of orthorhombic structures of 1D ordered polymeric

chains. These structures are known as ‘‘low-pressure ortho-

rhombic’’ (P � 1.5 GPa) or ‘‘high-pressure orthorhombic’’

(P > 2 GPa) phase (Table 2) [84–86]. The 1D polymeric chains

can form along the six <110> cubic directions, and are char-

acterized by different spatial orientations around their axes,

usually called ‘‘lying’’ or ‘‘standing’’ orientation referring to

the spatial orientation of the chain link (Fig. 9).

The ‘‘high-pressure orthorhombic’’ structure, labelled O

(Immm pseudo-tetragonal, because aO and bO are almost

equal) has been obtained at 2 GPa and 573 K and it was similar

rs obtained from a P–T preparation path and the

c (A) V (A3) P (GPa) T (K) l Refs.

14.22 650 2.0 573 0� or 90� [84]14.72 658 1.5 723 0� or 90� [85]14.66 663 1.2 585 29� [86]

– – 61� [90]– – 29–31� [91]

y the angle l formed between the chain link (four-membered

ions. The chains are normal to the figure and run along

ers (2D polymers, see Section 6) that can be obtained

. Reprinted with permission from Moret et al. AIP Conf. Proc.

C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7 389

to a previously known phase formed at higher pressures

(8 GPa and 573 K) [5,84]. The structural model was based on

the 1D polymer structure formed in alkali doped fullerenes

(A1C60 with A = K, Rb or Cs) [87–89]. C60 chains can adopt dif-

ferent orientations (Fig. 10) in the Immm space group symme-

try. This is a consequence of the spatial configuration of the

four-membered rings that can be either in the (aO, cO)-plane

(l = 0�) or in the perpendicular (aO, bO)-plane (l = 90�).The synthesis of a ‘‘low-pressure orthorhombic’’ phase

labelled O 0 at 1.5 GPa and 723 K was claimed 2 years later

[85]. The diffraction spectra of O 0 samples showed better

resolved Bragg peaks than those of O, and this was inter-

preted as a sign of a less disordered phase or a more homoge-

neous polymerization. The O 0 diffraction pattern could be

indexed either with an orthorhombic symmetry (Immm) or a

rhombohedral (R-3m) one. The lattice parameters obtained

from the fit, were similar to those of the 2D-tetragonal phase

(T) (see Section 6 for details) reported earlier [84], Agafonov

et al. suggested phase O 0 as an intermediate stage to obtain

the tetragonal phase [85].

The first single crystals of phase O 0 were synthesized at 1–

1.2 GPa and 550–585 K [86]. Their diffraction pattern exhibited

Bragg spots from different orientation variants (multi-domain

crystals) which indicated that pseudo-cubic symmetry

Fig. 10 – C60 polymer structure for different orientations of the ch

(l = 90�) and (c) P42/mmc polymer structures. In (c) the four-mem

plane of the figure while in (a) are all parallel and in (b) all are p

elements are present through the transition of cubic C60

monomer to a low symmetry phase. For their samples, the

authors proposed the Pmnn space group that allows for two

alternative chain orientations (tilted by an angle ±l). Two

chain orientation models (l = 45� and l = 29�) were used for

the experimental determination of the chain orientation from

the diffraction data, but the fit did not show a clear tendency

towards any of them. Energy-calculations performed for the

different packing of the orthorhombic cell (O and O 0) reported

the most probable configuration for the 1D chains as Pmnn

with l = 29–31� [90–92]. Structural investigations of C60 phase

transitions at 1.5 GPa concluded that the 1D reaction starts

with the dimerization of the C60 molecule at room tempera-

ture. Certain arguments have advanced that on increasing

temperature the dimers become chains of the O 0 phase that

slowly transform into layers of the 2D-tetragonal phase (T)

[93]. The authors indicated that the pure T phase should be

the more stable one at 1.5 GPa in the 723–900 K range, but

so far no in situ experiments have been performed in this

region of the P–T diagram and no evidence of a pure T phase

has been reported. Therefore, a reinvestigation of the trans-

formations occurring in the low-pressure region is still

needed. Such an experiment is also important to better

understand the process of chain cross-linking that occurs at

ains. Projections along baxis of the (b) Immm (l = 0�), Immm

bered rings are alternately perpendicular and parallel to the

erpendicular.

390 C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7

higher pressures and temperatures, leading to the 2D poly-

merization within the {100}cubic and {111}cubic layers of

the parent cubic monomer structure.

5.2. Chains of 1D confined fullerenes

Single-walled nanotubes (SWNT) offer a 1D confined nano-

sized space (typically 1.4 nm) which might act as mould, reac-

tor or container for preparation of hybrid nanomaterials such

as X@SWNTwhere X can be atoms, molecules or compounds,

as reviewed in [8]. Research in this area is promising for the

creation of new molecular devices. Novel physical properties

(e.g. mechanical, transport, electronic) are expected due to

the nanometric dimensions of the system, the electronic

interactions of the chemical species with the outer tube and

the polymerization or coalescence of the confined molecules.

C60 molecules encapsulated in SWNT, so called peapods,

were discovered as low quantity by-products in nanotube

preparation via pulsed laser vaporization, purification and

annealing (Fig. 11) [94,95]. Synthesis of larger quantities of

C60 peapods is now accessible, opening ways to investigate

the influence of 1D confinement in the behavior of fullerenes

Fig. 11 – High resolution TEM image of C60@SWNT’s. The

nanotube diameter is (10,10). Electron irradiation promotes

dimerization of C60 molecules, which subsequently tend to

arrange in easily distinguishable pairs (an example is

arrowed). Reprinted with permission from Smith et al.

Chem Phys Lett 1999; 315: 31–6 [96]. Copyright (1996), with

permission from Elsevier.

Fig. 12 – Coalescence of C60 molecules. Left-panel images includ

depicted for visualization purposes. The middle panel shows th

experimental sequence of C60 peapods under irradiation. The ca

a corrugated tubule. Reprinted with permission from Hernande

American Chemical Society. (A color version of this figure can b

and other fillings [96]. Polymerization or coalescence mecha-

nisms acting on confined 1D arrays of C60 molecules are of

high interest for their use as preparation methods for derived

materials with tuned electronic properties. Structural transi-

tions and electronic properties of peapods have not yet been

fully understood. For example, superconductivity has been

predicted to occur in alkali-doped C60 peapods [62–64].

5.2.1. Coalescence of confined C60

Thermal annealing [95] or electron irradiation [97] can induce

fullerene dimerization followed by coalescence (e.g. above

�800 �C in [98]) creating corrugated tubules with very small

diameters (e.g. 4–7 A) containing pentagonal, hexagonal, hep-

tagonal and octagonal rings (Fig. 12) [97]. Theoretical

approaches indicate that the thermal mechanism for fuller-

ene coalescence is driven by surface-energy minimization

whereas the radiative one proceeds by the creation of dan-

gling bonds and vacancies. The SWNT remains almost intact

while C60 molecules are preferentially damaged due to differ-

ences in strain energy [97]. Coalescence temperature can be

lowered by combining heating and irradiation [99] while con-

ditions for preventing structural damages are reported in

[100]. Further heating (e.g. �1200 �C [98]) or electron irradia-

tion [100] transform the tubules into double-walled nano-

tubes (DWNT) [101] as evidenced by Raman [98] and

transmission electron microscopy (TEM) for C60 and other

higher-order fullerenes [95,98,102]. The transformation of

C60 peapods into DWNT’s implies re-orientation and coales-

cence via cycloaddition processes and Stone–Wales re-

arrangements [[102] and references therein].

5.2.2. Polymerization of confined C60

Polymerization of C60 peapods can be achieved at room tem-

perature by irradiation [103,104] or doping [105] and also by

high-pressure-high-temperature treatment [106,107]. Room

temperature polymerization indicates that C60 molecules

e MD simulations, note that the outer (10, 10) tube is not

e HRTEM simulations and the right panel shows the

rbon cages (b) polymerize, (c) rearrange, and (d) coalesce into

z et al. Nano Lett 2003; 3(8): 1037–42 [97]. Copyright (2003),

e viewed online.)

Fig. 13 – The center-to-center distance of the nearest C60

molecules in C60 peapod s2 (circles) and f.c.c. C60 (triangles)

samples as a function of pressure. Filled and open circles

represent data with increasing and decreasing pressure,

respectively. Reprinted with permission from Kawasaki

et al. Chem Phys Lett 2006; 418(1–3): 260–3 [107]. Copyright

(2006), with permission from Elsevier.

Fig. 14 – Pressure–temperature reaction diagram of C60

peapods. Open symbols (Ds·) and crosses refer to C60

monomers, while solid symbols (mjd) denote polymers.

The star symbol corresponds to the formation of DWNTs.

The dashed area is a tentative representation of the area

where polymerization can occur in C60 peapods. Reprinted

with permission from Chorro et al. EPL 2007; 79(5): 56003

[[109] and references therein]. Copyright (2007), IOP

Publishing.

C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7 391

rotate inside the nanotubes [104]. Inelastic neutron scattering

performed at room temperature and pressure in as-synthe-

sized peapods, suggest that the SWNT are filled with a mix-

ture of monomer and n-mers (dimers, trimers and short

polymers) in variable proportions [73]. The activation energy

needed for the formation of polymers is provided by the

kinetic energy gained after encapsulation [108]. Quasi-free

rotational diffusion of C60 molecules inside SWCNTs is

observed until a very low temperature (100 K 6 Tconfined -

6 200 K) [74], which is much lower than To � 260 K, where

unconfined C60 undergoes an orientational order transition

[27]. Therefore, C60 molecules inside nanotubes can re-orient

themselves to favor the occurrence of [2 + 2] cycloadditions.

For confined C60, polymerization can be achieved by press-

ing-then-heating the as-prepared peapods [106,107]. The pro-

cess has been investigated by in situ X-ray diffraction (XRD)

under high pressure up to 25 GPa. The C60–C60 distance

decrease rapidly as function of pressure down to a value of

8.45 A around 10 GPa, which is short enough to be considered

as polymerized C60 (Fig. 13). The bonding is preserved after

the release of the pressure [107] and the C60–C60 distance is

reduced from 9.8 A to 9.1 A (e.g. 1.5 GPa and 300 K [109]), as

in unconfined 1D polymers [90,91]2.

For comparison, the same treatment than [106] (e.g. 4 GPa

and 1023 �C) applied on unconfined C60 monomer leads to the

formation of 2D polymers and subsequent transformation

into disordered corrugated graphene-like carbon is induced

2 A particular formalism which considers two preferential orientsymmetry (e.g. C60@SWNT ‘buckypaper’ [106]) has been developed indistances) from the diffraction spectra of pressure oriented peapods

above �1000 �C [111,112]. The P, T region where 1D polymers

exist is extended for peapods as compared to unconfined

C60, as shown in their reaction diagram (Fig. 14).

The polymerization process requires thermal activation to

increase the probability that confined neighboring molecules

adopt a favorable orientation to undergo [2 + 2] cycloaddi-

tions. Under a pressure around 2–2.5 GPa the nanotubes suf-

fer ovalization [113], while the C60 filling still acts as

reinforcement avoiding nanotube destruction [106]. Other

transitions might take place depending on the choice of the

filling and the pressure transmitting medium. It has been

found that for C70 filling, flattening of the empty tubes and

deformation of peapods takes place in the 10–30 GPa range

[113].

Raman measurements evidence that doping the C60 pea-

pods with potassium leads to the formation of metallic poly-

mer chains C60�6 (Ag(2) mode at 1428 cm�1) instead of a

semiconducting polymer. The process start with charge

transfer from the K atoms to the SWNT and for heavily doped

state the C60 molecules are included in the process. The shift

on the Ag(2) mode is consistent with the formation of addi-

tional covalent bonds in the charged polymer in contrast with

the ions on K6C60 (Ag(2) mode at 1432 cm�1) due to competi-

tive charge transfer between the SWNT and the C60 chains.

Additional lines at 370 and 620 cm�1 correspond well with

the 1D orthorhombic phase of RbC60 [105].

Pressures and temperatures where polymerization of C60

peapods occur are consistent with those of unconfined C60

molecules (e.g. 1.5 GPa and 300 K), whereas 1D polymerization

of C70 peapods is not observed, even at higher P, T conditions

ations of tubes: 1D (fiber) and 2D (pellet) both having an axialorder to extract structural parameters (lattice parameter, C60–C60

[110].

392 C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7

for which unconfined C70 has been found to polymerize

[109,114]. Confinement of C70 molecules strongly constraints

their reactivity by limiting the orientational and translational

positioning of these ovoidal molecules The different orienta-

tions that can be adopted by the C70 molecules – molecule

long axis parallel (lying) or perpendicular (standing) to the

tube axis, are strongly influenced by the specific diameter of

the nanotube (1.42 nm being the transition diameter from

lying to standing orientations) [[115] and references therein].

6. 2D polymers and derived carbon phases

In the pressure range P � 2–8 GPa and in the temperature

range between 500 and 900 K, 2D polymers formed by tetrag-

onal (T) or rhombohedral (R) layers of polymerized C60 mole-

cules have been reported [84,116]. For pressures between 2

and 4 GPa, several authors report mixtures of R and T phases,

whereas at higher pressures of 4–8 GPa, mainly R phases are

observed. Each polymeric structure is discussed indepen-

dently in the next sections.

6.1. 2D tetragonal phases

Tetragonal polymerized layers are generated by additional

four-membered rings crosslinking the C60 chains within the

{100} cubic layers. Therefore, the 2D-T layers are stacked

along the former <100> cubic directions. The crystallographic

relationships between the original f.c.c. cell and the tetrago-

nal cell correspond to aT = bT = aC =p

2/2 and cT = aC (Fig. 15).

Fig. 15 – Crystallographic relationships between the f.c.c. cell an

(b) the tetragonal (aT, bT, cT) and cubic axes (aC, bC, cC) are related a

version of this figure can be viewed online.)

Table 3 – Crystallographic data for the 2-D tetragonal polymerspressing (T–P) preparation paths.

Path Report on Phase Space Group

P–T Powder 35%T–65%R ImmmPowder 65%T 0–35%R P42/mmcSingle-crystal T Immm

T–P Powder 90%T 0 P42/mmcSingle-crystal 75%T 0–25%R–some

dimersP42/mmc

Single-crystal 84%T 0–16%T 84% P42/mmc 16% Im

So far, a pure tetragonal phase has not been obtained. Only

mixtures of T and R polymers (Section 6.2), with not less than

10%R in quenched powder samples produced via pressing-

then-heating path (Table 3). Nunez-Regueiro et al. produced

the first samples of these mixtures (R:T – 65:35) at 3 GPa and

873 K [84] and proposed a tetragonal structure, labelled T

(Immm, with aT = bT). Davydov et al. proposed an alternative

structure, labelled T 0 (P42/mmc) for mixtures (R:T – 35:65) pre-

pared at 2.2 GPa and 873 K [117]. In the Immm structure, all the

four-membered rings in adjacent stacked layers have the

same spatial orientation (all perpendicular) whereas in the

P42/mmc structure they are alternately parallel and perpen-

dicular, related by a 90� rotation around the cT-axis of the

tetragonal cell (Fig. 16). Dzyabchenko et al. performed

energy-calculations for the different packing of the tetragonal

cell (T and T 0) and determined that the P42/mmc stacking was

more stable than the Immm stacking [118]. The Immm struc-

tural model was confirmed later by Chen and Yamanaka from

X-ray diffraction of single crystals prepared at 2.5 GPa and

773 K [119].

Indeed, the P42/mmc structure is easily obtained if a heat-

ing-then-pressing path is used for sample synthesis. In this

way, powder samples [120] and single crystals [119,121–123]

with enhanced content of T 0 phase (�90%) have been pre-

pared. The single crystals from Moret et al. were found to

be mixtures of T and R polymers (R:T – 25:75) with some

dimers [123]. Those of Narymbetov et al. were fitted by a

model containing a mixture of T and T 0 phases (84% P42/

mmc and 16% Immm) [122]. Chen and Yamanaka (2002)

d the tetragonal cell. (a) 2D tetragonal polymerized layer and

s follows: aT = (aC + bC)/2, bT = (�aC + bC)/2 and cT = cC. (A color

obtained via pressing-then-heating (P–T) and heating-then-

a (A) b (A) c (A) V (A3) P (GPa) T (K) Refs.

9.09 9.09 14.95 618 3 873 [84]9.097 9.097 15.04 622 2.2 873 [117]9.026 9.083 15.07 618 2.5 773 [119]

9.097 9.097 15.04 622 2.2 873 [117,120]9.02 9.02 14.934 607 2.0 700 [121,123]

mm 9.064 9.064 15.039 618 2.2 873 [122]

Fig. 16 – Structural models of the 2D tetragonal phases. (a) the C60 tetragonal layers are stacked along the vertical cT direction

(originating from the a (b or c) direction of the cubic structure of the C60 monomer) (b) the Immm structure proposed by Nunez-

Regueiro et al. Phys Rev Lett 1995; 74(2): 278–81 [84] (c) the P42/mmc structure proposed by Davydov et al. Phys Rev B 1998;

58(22): 14786–9 [117].

C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7 393

obtained only bad quality twinned crystals, unfortunately not

suitable for a complete resolution of the structure [119]. Ben-

nington et al. performed the unique available in situ X-ray dif-

fraction study in energy-dispersive mode at 2.6 GPa and 300–

1130 K [124]. These authors reported the coexistence of the

f.c.c. and the T structures up to 1100 K where they indicate

that the material is almost completely tetragonal, unfortu-

nately the corresponding diffraction pattern together and its

indexation is not shown in their publication.

6.2. 2D rhombohedral phases

Rhombohedral polymerized layers are generated by addi-

tional four-membered rings crosslinking the C60 chains

within the {111} cubic layers. Therefore, the 2D-R layers are

stacked along the former <111> cubic directions. The crystal-

lographic relationships between the original f.c.c. cell and the

rhombohedral one corresponds to aR = bR = aCp

2/2 and cR = aC-p3 (Fig. 17).

Iwasa et al. obtained the first structures of this type at

5.0 GPa and indexed them as f.c.c. (a = 13.6A) at 573–673 K

and R (R-3m) at 773–1073 K (Table 4) [116,125]. Similar struc-

tures were found later by Blank et al. at 6.5–7.5 GPa and 700–

900 K [126]. Nunez-Regueiro et al. obtained mixtures of R

and T phases (R:T – 82:18) at 4.0 GPa and 973 K and proposed

a model of ABCABC stacked layers were contact between mol-

ecules involves pentagons (model I) [84]. Their model was val-

idated by the theoretical work of Xu and Scuseria [127] and by

Oszlanyi and Forro [128]. Davydov et al. also produced R and T

mixtures (R:T – 85:15) by lowering the pressure from 6.0 GPa to

2.2 GPa at 873 K [117]. From these samples, a second struc-

tural model of ACBACB stacked layers was proposed by

Dzyabchenko et al. [118]. It involves a 60� rotation of the C60

molecules around their 3-fold axis with respect to the previ-

ous structure, therefore in this model the contact between

the molecules only involves hexagons (model II) (Fig. 18). A

third model of combined alternated layers (model III) was also

considered by Davydov et al. [129]. The second structure was

experimentally observed in single crystals produced by Moret

et al. via a heating-then-pressing process [86,121]. Chen et al.

reported single crystals exhibiting mixtures of T and R orien-

tational domains and disordered dimers [119,130]. Recent

density-functional theory studies found similar energies (by

�0.01 eV/C60) for both models (I and II) [131,132].

Furthermore, Marques et al. reported structures obtained

at 4.8 GPa and identified them as orthorhombic (O) at 573 K

and rhombohedral (R) at 873 K [133]. Sundar et al. obtained

samples that were also indexed with an orthorhombic struc-

ture [134]. They were crystallized under long treatment

(around 6 h) at 5.0–7.5 GPa and 623–773 K [134]. Szwarc et al.

highlighted a distortion of the f.c.c. lattice between 400 and

700 K, interpreted as an f.c.c. mixture with high-pressure

phases, and also proposed a monoclinic single-phase index-

ation for the R and T mixtures [135]. Blank et al. reported mix-

tures of orthorhombic (O) and rhombohedral (R) phases at

8 GPa and 720–920 K [3]. During their in situ energy-dispersive

diffraction studies at 5.7 GPa, Bennington et al. reported the

appearance of a R structure at 650 K, directly from the f.c.c.

structure and above 1000 K, they also observed a disordered

graphite-like structure [124].

6.3. Corrugated anisotropic graphene-based carbonsderived from 2D polymers

When C60 polymers are exposed to temperatures above

1000 K, a structural phase transition takes place. So far it

was assumed that C60 molecules ‘‘collapsed’’ (cage break-

down) or ‘‘amorphized (destruction of the C–C bonds) into dis-

ordered phases. The obtained phases are referred in the

literature as disordered graphite-like carbon (DGLC). Very

recently, light has been shed on the structural organization

of these phases, showing that they are more like corrugated

anisotropic graphene-based carbons and outlining the crys-

tallographic relationships that link them to the 2D parent

polymer phases [111,112,136]. Exceptionally, we choose the

term DGLC here to facilitate correlation with previous reports.

DGLC phases exhibiting high hardness are obtained in the

2–8 GPa range, by high temperature treatment (T � 1000 K) of

Fig. 17 – Crystallographic relationship between the f.c.c. cell and the rhombohedral cell. (a) 2D rhombohedral polymerized

layer and (b) the rhombohedral (aR, bR, cR) and cubic axes (aC, bC, cC) are related as follows: aR = (aC � bC)/2, bR = (bC � cC)/2 and

cR = aC + bC + cC. Tetrahedra faces are aligned along the {111} cubic crystallographic planes. (A color version of this figure can

be viewed online.)

Table 4 – Crystallographic data for the 2-D rhombohedral polymers obtained via pressing-then-heating (P–T) path and thecorresponding theoretical calculations.

Report on Phase Space Group a (A) c (A) V (A3) P (GPa) T (K) Refs.

Experimental dataPowder R R-3m 9.22 24.6 603 5–7.5 773–1073 [125,126]Powder 18%T–82%R R-3m model I 9.19 24.5 597 4–5 873–973 [84,133]Single-Crystal 75%T–25%R–some dimers R-3m model I 9.19 24.5 597 2.2 700 [121,123]Powder R R-3m model II 9.175 24.568 597 6 873 [91,92]Single-Crystal R R-3m model II 9.175 24.568 597 5 773 [119,131]

Theoretical calculationsTheory R R-3m model III 9.17 49.0 [117]

394 C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7

the 2D-polymers [137–140]. In the existing literature DGLC

samples are considered to be predominantly sp2-based

phases that exhibit semi-metallic properties (i.e. as graphite).

High hardness, which is still under debate, has been reported

for DGLC phases [137,141,142]. Certain specimens of DGLC

have also been described as nearly amorphous [142], similar

to amorphous fullerites prepared by heat-treatment under

vacuum [143] or by mechanical milling [144] without deeper

explanation on the DGLC structure.

So far, experiments have focused on the correlation

between the density and the mechanical properties of DGLC,

which are claimed to be hard-carbon phases [137,141,142]. For

Fig. 18 – Structural model of the 2D rhombohedral phases. Projection of the C60 rhombohedral layers along the rhombohedral

[001] direction: (a) model I proposed by proposed Nunez-Regueiro et al. Phys Rev Lett 1995; 74(2): 278–81 [84] and (b) model II

proposed by Dzyabchenko et al. Crystallogr Rep 1999; 44: 13–7 [118] and (c) model III proposed Davydov et al. J Chem Phys

2001; 115(12): 5637–41 [129]. The polymerized layers are stacked along the vertical cR direction (equivalent to the [111]

direction of the cubic structure of the C60 monomer). A–C denote the stacking of C60 molecules in successive layers. The

pentagonal faces are shadowed, to clarify the orientation of the molecules between the two arrangements. In model III the

length of the c-axis is multiplied by 2.

C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7 395

instance, the density of DGLC (1.9–2.3 g/cm3) is reported sim-

ilar to graphite (2.26 g/cm3) or to amorphous carbon (1.8–2.1 g/

cm3) [138,139]. In contrast, the measured Vickers’s hardness

of DGLC (10–40 GPa) is reported to be higher than graphite

(0.2 GPa) or the parent 2D-polymers (1–2 GPa) (Fig. 19) [137–

139,145].

This peculiarity has been explained by the possible occur-

rence of cross-links between the chains and planes in 1D and

2D polymers at high pressure. Moreover, under indentation,

DGLC phase shows a high degree of elastic recovery

[136,145], similar to other carbon-based systems (i.e. films of

CNx [146], carbon-onions or carbon-nanotubes [147]). For

comparison, the structure of CNx films for example, is consid-

ered to be a network of buckled sp2-hybridized CNx planes,

cross-linked by sp3-hybridized bonds. In the case of DGLC,

hardness and elastic properties have been attributed to a pos-

sible corrugated character of the graphitic layers that could

allow them to follow compression-expansion cycles. In addi-

tion, the elastic properties of DGLC exhibit directional anisot-

ropy [148,149]. X-ray diffraction patterns along and across the

loading axis show a very pronounced directional texture, indi-

cating preferred orientation of pseudo graphite-like domains

at the meso-scale level together with phase heterogeneities

at different spatial scale-levels [111].

Early studies on DGLC demonstrate that ED patterns

exhibited textured pseudo-graphitic broad reflections with

inter planar distances that are smaller than those between

planes in perfect hexagonal graphite (d002 = 3.35 A) [137].

These reflections can show either one-folded (labeled as a

or b) or two-folded (a and b) texture depending on the sample

orientation (Fig. 20) [140]. At the time, the explanations for the

texture features remained open.

Additional studies show that ED patterns of DGLC samples

[112] show two continuous diffuse rings at 2.1 A (01DGLC) and

1.2 A (11DGLC) corresponding to hk0 basal-plane reflections

and also textured 00lDGLC inter-layer broad reflections which

can be compared to the 002graphite and 004graphite inter-layer

reflections in perfect hexagonal graphite. For basal-plane

reflections, the continuous hk rings indicate a turbostratic-

type disorder between successive carbon sheets. Their diffuse

and broadness character indicate small grain size, local in-

plane deformation and/or disorder in atomic positions. For

textured inter-layer reflections, analysis of DGLC samples

synthesized at different pressures between 2 and 8 GPa dem-

onstrate that 00lDGLC broad arch-shaped textured reflections

make mainly angles of 55�/70�, 60� but also 90�, as well as con-

tinuous diffuse 00lDGLC rings (Fig. 21). HRTEM images show

distorted and corrugated carbon sheets (Fig. 5.10), which is

consistent with the observed broad arc-shape reflections.

According to Alvarez-Murga et al. [111,112], this layered

carbon phase is composed of corrugated layers having a

pseudo-epitaxial relationship with the {111}cubic planes of

Fig. 19 – Dependence of the density and Vicker hardness vs.

synthesis temperature, for phases prepared at 3.5 GPa

(triangle), 5.0 GPa (square) and 8.0 GPa (circles). Open

symbols correspond to the polymerized phases of C60 and

closed symbols to DGLC samples. The lines are a guide for

the eye. Reprinted with permission from Lyapin et al. Appl

Phys Lett 2000; 76(6): 712–4 [139]. Copyright (2000), AIP

publishing LLC.

396 C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7

the parent polymer phase. The authors propose a pseudo-

martensitic transformation mechanism with a partial C–C

bond reconstruction, as indicated by measurements of the

angles between textured 00l reflections. The distribution of

dense polymeric planes in tetragonal, rhombohedral and dis-

torted pseudo-cubic 2D polymers, as well as the crystallo-

graphic relationships between different layered carbon

microstructures indicate that these carbon layers are gener-

ated from the polymerized dense planes of each structure.

Analysis of the structural organization and transformation

mechanism of 2D-polymers into DGLC, requires to analyze

Fig. 20 – Electron diffraction pattern (a) and high resolution TEM

the sample are 8 GPa and 727 K. Reprinted with permission fro

[140]. Copyright (2005), IOP Publishing.

partially transformed samples and probes that cover different

scale ranges. Few studies have achieved this by combining

transmission electron microscopy (TEM), electron diffraction

(ED) and X-ray diffraction (XRD) [112,140,145] and novel 3D

structural imaging methods [111]. Expanding the knowledge

of the structural organization of DGLC is important for: (a)

improving the comprehension of the C60 and C phase dia-

grams, (b) clarifying the transformation mechanism from 2D

polymer (R or T) to DGLC, (c) interpreting and tailoring the

DGLC physical properties (i.e. elastic, transport and hardness)

and (d) preparing novel hard carbon nanomaterials [136].

7. 3D polymers and derived carbon phases

At pressures above 8 GPa combined with elevated tempera-

tures formation of 3D polymers with crystalline and disor-

dered structures have been reported. The 2D polymerized

layers get linked in the stacking direction through additional

cycloaddition reactions to form 3D polymerized structures.

The resulting crystalline phases are considered to be predom-

inantly 3D networks preserving the C60 molecules. These

materials behave like ultra-hard wide-bandgap semiconduc-

tors (i.e. as diamond) [150]. However, the details of this type

of polymerization are still unclear as most of the samples

obtained are inhomogeneous and contain at least two phases.

7.1. First evidence of 3D polymers

The first bulk samples of 3D polymerized C60 were produced

by Blank et al. in the range of 9.5–13.0 GPa and 300–2100 K

(Fig. 22a), and several symmetries were reported to fit their

experimental data [126,141]. At 9.5 GPa the authors reported

that the f.c.c. structure was preserved up to 650–700 K. The

cubic lattice parameter (a), determined on quenched samples

exhibits an inverse dependence with temperature (Fig. 22b).

An important volume variation could be observed around

520 K (a � 13.6 A). Under these conditions, the C60 cages are

preserved, although C–C intermolecular distances are

reported to decrease from 2.97 A (at 300 K) to 1.86 A (at

670 K), suggesting a 3D polymer of sp3 bonded molecules

[126,141]. The authors interpreted this observation as a fron-

tier between soft and hard phases (i.e. hard: Hv > sapphire,

super-hard: Hv > cubic BN and ultra-hard: Hv > (100) diamond

image (b) typical for the DGLC. The synthesis conditions of

m Tat’yanin et al. J Phys Condens Matt 2005; 17(2): 249–56

Fig. 21 – The 00lDGLC appears either as textured reflections making angles of 55�/70� (2D rhombohedral polymer), 60� (2D

tetragonal polymer) and �90� (3D cubic polymer); or as a continuous diffuse ring (3D cubic polymer). The crystallographic

relationships between DGLC and both, tetragonal (60�) and rhombohedral (55� and 70�) polymers, indicate that DGLC planes

are generated from the dense polymerized planes of each structure. Reprinted with permission from Lepoittevin et al. Carbon

2013; 52: 278–87 [112] Copyright (2013), with permission from Elsevier.

Fig. 22 – (a) Sequence of X-ray diffraction patterns of C60 quenched from 13 GPa and temperatures 300–2100 K. (b) Temperature

dependence decrease of the unit-cell volume V for f.c.c. (open squares), distorted b.c.c (closed squares) and monoclinic

(circles) cell parameters. Reprinted with permission from Blank et al. Phys Lett A 1995; 205(2–3): 208–16 [141]. Copyright

(1995), with permission from Elsevier.

C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7 397

and (111) diamond faces). At 800 K, they reported formation

of a disordered ultra-hard structure (see Section 7.2 for

details) and above 800 K, the authors indexed their patterns

using a f.c.c. cell (a � 11.75 A) or using an hexagonal cell

(a � 11.44 A, c � 8.33 A). At 13.0 GPa the f.c.c. structure was

reported as conserved up to 900 K (a � 12.2 A) [141,151],

398 C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7

together with a secondary b.c.c. phase (a � 11.16–11.22 A,

b � 8.17–9.05 A, c � 7.58–7.68 A) that distorted into a mono-

clinic symmetry (a � 8.77 A, b � 12.48 A, c � 7.59 A, b = 110.6�)at even higher temperatures [126]. Brazhkin et al. reported

similar results of phase coexistance between f.c.c. (a � 12.1–

12.3 A) and monoclinic structures (a � 11.8 A, c � 13.6 A) at

12.5 GPa in the 573–773 K range [151]. Finally, at higher tem-

peratures (800 K in [151] and 1300 K in [141]) 3D-C60 was

reported to transform into a disordered ultra-hard phase

exhibiting either amorphous-like patterns or very few broad

reflections. It should be noted that information extracted

from the X-ray diffraction spectra is limited, as only a few

overlapping and broad reflections are available, which in turn

explain the variety of symmetries that could fit the experi-

mental spectrums (Fig. 22a). Complementary structural meth-

ods and calculations have been employed later to

discriminate between the structures (Section 7.2).

Raman studies of these phases show additional lines at

960 cm�1, which are interpreted as a characteristic of four-

membered rings stretching mode. These authors [141,151] con-

sidered this feature as a sign of formation of extra sp3 bonds

between neighboring molecules, similar to previous observa-

tions done on chain polymerized molecules of RbC60 [152].

According to this result, Brazhkin et al. proposed a model of

three-dimensional polymerization (P � 12.5 GPa) in which the

lattice parameter is a continuous function of the fraction of

covalently bonded molecules. They reported that the period

a � 13.8 A is the threshold for the formation of a 3D rigid poly-

mer whereas a � 12.3 A leads to the formation of an amor-

phous network with a high degree of sp3�bonding [153,154].

In other hand, Davydov et al. prepared samples at 10 GPa and

1900 K and proposed polycondensation as another type of

reaction that could form 3D structure [155]. Chernozatonskii

et al. proposed a different description of these structures based

on a transformation of the spherical C60 molecules into ‘‘bar-

rels’’ and the polymerization of the latter [156].

Later, Bennington et al. reported an in situ f.c.c. lattice

parameter of 12.5 A at 12 GPa, supporting a random bond

Fig. 23 – Difraction patterns (a) and unrolled projection (b) showi

obtained when the beam direction is nearly perpendicular to th

entire ring corresponds to a 360� variation of the azimuthal ang

difference respect to a perfect line at a given 2h angle. From Ma

permission from AAAS. (A color version of this figure can be vi

formation between the molecules, in all directions [124]. In

the f.c.c. model, each C60 molecule in the polymer is linked

to the 12 nearest neighbors by [2 + 2] cycloaddition between

the common pentagon-hexagon edges. To analyze symme-

tries of these 3D polymers Marques et al. studied them by

exploiting the information contained in the 2D diffraction

patterns and not only 1D integrated diffraction patterns. They

observed Debye–Scherrer ellipses on 3D polymerized samples

prepared at 13 GPa and 820 K (Fig. 23) [14]. They correlated the

elliptical character of the Debye–Scherer rings with the huge

anisotropic deformation (Da/a = 9%) induced by the sample

preparation (e.g. nonhydrostatic compression/uniaxial pres-

sure) and this huge deviatoric stress provided an unambigu-

ous probe of polymerization favored along pressure

direction, giving rise to an oriented/ordered polymer (Fig. 23)

[14]. This observation can explain the multitude of symme-

tries observed by using only 1D diffraction patterns (as seen

in Section 7.1). This observation and mechanism is not

restricted to the 3D phases but is a more general feature

occurring in C60 polymers [158]. Mezouar et al. extracted a

high bulk modulus of 288 GPa from the fitting of the unit-cell

(pseudo f.c.c) volume to a Vinet equation of state and con-

firms the very high stiffness and anisotropy of this low-den-

sity material [159]. Two different linear compressibilities of

231 and 320 GPa were obtained along the axial and the radial

directions, respectively. Although this values remain lower

that the bulk modulus of diamond (B0 � 441 GPa), the sp3

character of the network is confirmed. Recently, 3D polymer-

ization of C60 has been achieved at lower pressures (6–7 GPa)

in the presence of CS2 as catalyst [157].

7.2. Structural models proposed for the 3D polymers

Due to the low number of reflections (only six) available in

their experimental data set, Marques et al. [14] used averaged

symmetries (cubic (3D-C), tetragonal (3D-T) and rhombohe-

dral (3D-R)) to perform their structural simulations. These

simulations gave similar agreements to the experimental

ng the elongated elliptical shape of the Debye–Scherrer rings

e uniaxial direction of compression during synthesis. An

le. The eccentricity of the ellipses can be estimated by its

rques et al. Science 1999; 283: 1720–3 [14], reprinted with

ewed online.)

Fig. 24 – Crystal structure of the starting (a) 2D-Immm

polymer and the resulting (b) 3D-tetragonal polymer from

Yamanaka et al. Phys Rev Lett 2006; 96(7): 076602 [167].

Copyright (2006) by the American Physical Society (http://

dx.doi.org/10.1103/PhysRevLett.96.076602). (A color version

of this figure can be viewed online.)

C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7 399

data. They confirmed the 3D bonding of C60 cages but they

could not unambiguously determine the local symmetry of

this phase and the individual carbon position. However, they

provide the lattice parameter for the three symmetries, which

are: 3D-C (a � 12.050 A), 3D-T (a � 12.430 A, c � 11.324 A) and

3D-R (a � 12.072 A, a � 93.445�).Later, Serebryanaya and Chernozatonskii proposed a

model for this 3D phase by using the six reflections of the

1D projection of this 2D diffraction pattern and by using a

body-centered orthorhombic (BCO) structure (Immm,

a � 8.67 A, b � 8.81 A, c � 12.6 A) with common four-sided

rings and (2 + 2) interfullerene cycloadditions in the layer,

and (3 + 3) bondings between the layers [156,160]. Other theo-

retical studies by Okada et al. predicted that 3D-BCO polymers

might be formed by the application of uniaxial pressure to the

2D tetragonal polymer [161]. According to DFT calculations

polymerization should take place at P � 20 GPa (cT � 10.7 A).

This polymerization will result in the formation of a stable

metallic phase having 24 sp3 and 36 sp2 hybridized C atoms

in each C60 molecule [161].

Another theoretical study, by Burgos et al., predicted that

uniaxial compression perpendicular to the chains in 1D

polymers or to the planes in 2D polymers could lead to 3D

polymerization around P � 14 GPa. They proposed to model

the 3D-BCO and 3D-BCC polymers with 52, 56 and 60 sp3

coordinated carbon atoms per C60 molecule [162]. Since then,

Fig. 25 – (a) Crystal structure of the 3D-rhombohedral polymer f

[169]. The structure in plane, where C60 units are linked by [3 +

bonded with the adjacent balls (green circles) by pentagon-hexa

Typical bond lengths are shown in (b). Copyright (2008) Americ

viewed online.)

several additional experimental studies have explored the

possibilities of a 2D conversion into 3D.

Meletov et al. submitted a sample of 2D tetragonal polymer

to high pressure and observed an irreversible transformation

of the material at 20 GPa [163]. The sample was inhomoge-

neous and contained both the new high-pressure phase and

disordered carbon phases. The authors highlighted the need

of in situ studies in order to clarify the transition. Another

experiment, by Leger et al. show that amorphization of the

2D tetragonal polymer occur gradually between 10 and

29 GPa when inter- and intra-layer distances become similar

[164]. Talyzin and Dubrovinsky performed an in situ Raman

study of the isothermal (800 K) compression of both tetragonal

and rhombohedral phases and showed that they transform

into ‘superhard’ phases at about 23 and 12 GPa, respectively

[165]. Furthermore, in situ X-ray measurements confirmed that

a first-order irreversible transition occurs around 24 GPa for

the tetragonal polymers, inducing a shortening of the c axis

[166]. This is interpreted as the signature of 3D polymerization

through deformation of the C60 molecules and cross-linking of

the tetragonal layers by [3 + 3] cycloaddition, in agreement

with Serebryanaya’s model [160]. More recently, Yamanaka

et al. synthesized a 3D tetragonal phase (Immm, a � 7.86 A,

b � 8.59 A, c � 12.73 A) by the topotactic conversion of 2D

tetragonal polymer single crystals at a pressure of 15 GPa at

873 K. Their results are supported by a model of 2D tetragonal

layers linked by [3 + 3] cycloaddition where the C60 molecules

are substantially deformed (cuboidal shape) (Fig. 24) [167,168].

Yamanaka et al. also obtained a 3D rhombohedral phase (R-

3, a = b � 8.4097 A, c � 21.3932 A) from samples of monomeric

f.c.c. C60 treated at 15 GPa and 823 K (Fig. 25) [160,169]. In this

model each C60 unit is bonded to 12 adjacent units. Six units

are bonded by [3 + 3] cycloaddition between adjacent penta-

gons in the plane perpendicular to the threefold axis and

another six units are bonded by pentagon–hexagon [2 + 2]

cycloaddition from the bottom and top of the plane. The main

difference between the 2D-R and 3D-R models is the distances

between the dense {111}cubic polymerized planes and the C60

molecular orientations, that determine the type and number

of C–C inter-molecular covalent bonds.

Recent tight-binding molecular-dynamics calculations

from Yamagami et al., show that the structural phase

rom Yamanaka et al. J Am Chem Soc 2008; 130(13): 4303–9

3] cycloaddition. The carbon atoms marked by red color are

gon [2 + 2] cycloaddition from the next layer as shown in (b).

an Chemical Society. (A color version of this figure can be

400 C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7

transition of the 2D-3D tetragonal occurs for a wide pressure

range 10–30 GPa [170]. The resulting 3D polymer phases only

show a small variation in the interfullerene bonding along

the ‘b’ axis, which is close to the observed experimental val-

ues [167]. The total-energy differences between these phases

are found to be small, of the order of 10 meV or even smaller.

In addition, the LDA geometry optimization suggests that the

model with four interfullerene bonds is the most stable con-

figuration and should therefore be observed experimentally.

Its energy per atom in the LDA is found to be relatively low,

and this phase should be stable once it was synthesized.

Since this 3D polymerized phase is found to be electronically

conductive, the phase is an interesting target in the field of

pressure synthesis of new materials. In 2011 the same group

of authors predicted the transition of s.c. C60 into a s.c. 3D

polymer with a high sp3 rate of 60% under 10 and 20 GPa

[171]. This s.c. 3D polymer is confirmed to be stable using

the local-density approximation (LDA). Interestingly, the lat-

tice constant of this s.c. 3D polymer is close to the experimen-

tal values observed by Yamanaka et al. [169] and Mezouar

et al. [159] although its detailed geometry is unknown.

7.3. Disordered diamond-like carbons derived from 3Dpolymers

Disordered sp2–sp3 carbon networks with enhanced sp3 carbon

content are obtained above 8 GPa by high temperature treat-

ment (T � 1000 K) of the 3D polymers (Fig. 26). These disordered

super-hard phases like those reported by Blank et al. at 9.5 GPa

and 800 K show extremely broad diffraction lines [126,141].

The structure labelled amorphous1 by Blank et al. [126] in

Fig. 26, was described by the authors as hexagonal (a � 2.38–

2.42 A, c � 6.5–6.2 A) and the line with maximum intensity

could be related to a d-spacing of 3.1–3.3 A, which is much

Fig. 26 – X-ray diffraction patterns of disordered super-hard ph

Brazhkin et al. J Appl Phys 1998; 84: 219–26 [172] Copyright (199

with permission from Blank et al. Phys Lett A 1996; 220(1–3): 14

(1996), with permission from Elsevier.

smaller than the d002 � 3.35 A of hexagonal graphite. How-

ever, only (h00), (00l) and (0k0) reflections present, whereas

all the others (hkl) reflections were missing. Above 1700 K,

the authors report that the quenched material contains

‘‘graphite-like’’ mixtures of phases that they called tran-

sient-states, as shown in Fig. 26. In contrast, the ultra-hard

disordered phases produced at 12.5–13.0 GPa and 800–1300 K

present different X-ray diffusive pattern than the previous

one (named amorphous2 in Fig. 26), with a maxima located

at a d-spacing of 2.18–2.16 A. Other observed experimental

distances corresponded to hk0 peaks (i.e. 1.25 A to (110)

and 1.08 A to (200)). The high hardness of this phase has been

interpreted as a 3D network with disordered sp3 bonds

[153,154,173]. Both phases, amorphous1 and amorphous2,

were proposed as different from previously reported amor-

phous structures (i.e. amorphous carbon, amorphous dia-

mond, collapsed fullerite [174–176]). Further heating, 1173 K

in [151] and in 1700 K [141], lead to the appearance of the

two new peaks [172]. These lines were later identified as the

(002) and (004) reflections from parallel-packed hexagonal

planes of graphite, although at lower temperatures the hexag-

onal layers were somewhat squeezed (d002 � 3.0A) [172].

Quantitative measurements of the mechanical properties

demonstrated that the hardness of crystalline or amorphous

3D polymerized phases of C60 obtained above 10 GPa is com-

parable to cBN (H � 40–60 GPa) [172]. However, the structure

and physical properties of these phases are still under open

debate. Brazhkin et al. have recently proposed an extended

version of the reaction diagram up to 20 GPa [15]. As the

authors mention themselves, the proposed diagram still

remain open to discussion as the structural knowledge of

the novel phases remains limited (Fig. 27). The particularity

of this diagram is that it includes recent results on the prep-

aration of sp3-based amorphous and nanocomposite phases

ases obtained at (a) 12.5 GPa. Reprinted with permission of

8), AIP publishing LLC. and (b) between 9–13 GPa. Reprinted

9–57, Phys Lett A 1995; 205(2–3): 208–16 [126,141]. Copyright

Fig. 27 – Suggested version of nonequilibrium transitional

pressure–temperature phase diagram of C60 up to 20 GPa.

The hard carbon structures indicated as nc–sp2 are

disordered (nanocrystalline) graphite-like phases; 3D (fcc)

are 3D polymerized C60 phases; a–sp3 and a(nc) � sp2 + sp3

are amorphous phases with possible nanocrystalline

inclusions in the second case. Extension proposed by

Brazhkin et al. Journal of Superhard Materials 2012; 34(6):

400–23 [15]. Copyright (2012), with kind permission from

Springer Science and Business Media.

C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7 401

obtained at very high pressures P12 GPa and high tempera-

tures. Mechanical properties of C60 derived hard carbon

phases have been reviewed and compared with other super-

hard materials [15].

8. Complexity, limitations and future of C60

analysis

We summarize here strong points that affect the analysis of

X-ray diffraction data from C60 ‘‘powder’’ samples submitted

to high pressure and high temperature treatment. The low

scattering power of carbon atoms and the complexity of the

sample environment require the use of highly energetic and

focused beams to perform in situ studies. Making an accurate

analysis using laboratory sources is highly restrictive. The

disordered, textured and multiphase character of the samples

is also a source of complexity. The limits here enounced are

present even when the latest methods including synchro-

tron-based in situ diffraction studies are employed.

Partial azimuthal scan: The complexity of the sample envi-

ronment limits the access to the sample and the information

that can be collected in situ. For example in a large volume

press the opening angle is limited to 15� in the vertical direc-

tion by the anvil geometry, which corresponds to the uniaxial

direction of compression during synthesis [177]. Therefore,

only partial azimuthal information is seen by the area detec-

tor during a scan in angle dispersive mode. The intensity

available in the equatorial plane is preferentially collected

whereas the one in the vertical direction is ‘masked’. The

analysis is incomplete, especially for samples developing tex-

ture [111] and anisotropy [14]. This is the case of C60, which

starts showing elliptical Debye–Scherrer rings and texture

already at the early stages of dimerization.

Background noise: In order to generate high pressures and

high temperatures the sample is confined in several cylindri-

cal concentric containers made of different materials (e.g.

boron nitride capsule, graphite furnace, boron-epoxy gasket).

Elastic and inelastic contributions from the sample environ-

ment generate a strong X-ray background. Reduce this back-

ground requires the use of a multichannel collimator as

spatial filter for the signal, but is done at the cost of rejection

of an important part of good data. Exploiting the intensities

for structural refinement requires a transfer function correc-

tion, especially for liquids and disordered samples [178,179].

Hydrostaticity: Hydrostaticity affects pressure-responses on

soft materials and layered compounds. This parameter

depends on the pressure medium but also the type of system

that is used to compress the sample. Reproducibility of the

preparation conditions (hydrostatic/non hydrostatic) remains

a challenge. Accurate measurements of the pressure and tem-

perature are widely adopted. Stress-driven effects [158] are less

clear, as the up to date reaction diagram summarizes experi-

mental data from samples prepared under different stress

conditions without distinction. C60 is particularly sensitive to

non-hydrostaticity and stress keeping a ‘deformation’ mem-

ory [14,180].

Multiphase: As discussed previously, C60 polymer samples

are often a mixture of at least two phases. Even the few sam-

ples of single crystals that have been produced show phase

coexistence multi-domains [86]. The relative content of the

constituent phases being dependent of the synthesis condi-

tions such as temperature, pressure, external stress (e.g. the

hydostaticity of the preparation cell), preparation path (T–P

vs. P–T) and reaction time.

Disorder and deformation: Asymmetric X-ray diffraction

peak broadening in C60 sign of disorder in the planes whereas

and defects. Disorder is present since the very early stages of

the dimerization process, creating peak broadening and over-

lap on the diffraction diagrams. Peak-deconvolution tech-

niques are necessary to analyze the ‘‘crystalline phases’’. For

higly disordered phases such as the ones derived from the

high temperature treatment of 2D/3D polymers X-ray pair dis-

tribution function analysis is imposed. In some cases, such as

3D polymers at pressures above 10 GPa mixtures of disordered

phases and polycrystals have been obtained. Analyzing this

type of sample require a combination of methods.

Theory: Expanding the theory concerning crystallographic

models for C60 polymers and derived phases, including pre-

dictions of phase transitions, physical properties and com-

pressibility would be highly appreciated [181]. Some derived

phases are graphene based-carbons containing some 6-, 5-,

4-folded carbon cycles. These phases can be recovered to

ambient conditions and could give rise to novel families of

carbon allotropes, such as the recent demonstrations for

polymerized carbon nanotubes [182].

8.1. Novel ways to push the limits of structural analysisfor carbon allotropes

Recently, novel X-ray diffraction/scattering based imaging

methods have become available. This synchrotron based

methods have potential impact across a range of disciplines

from materials science, biomaterials, geology, palaeontology,

environmental science, and cultural heritage. Local, high

resolution, phase selective analysis can be performed on

402 C A R B O N 8 2 ( 2 0 1 5 ) 3 8 1 – 4 0 7

multiphased, heterogeneous and disordered samples. The

results are rendered as 3D maps of phase distribution, con-

tent and residual stress. We briefly mention them here

including references for detailed description of the experi-

mental protocol. Diffraction/scattering computed tomogra-

phy is useful for three-dimensional characterization of

multi-phase crystalline and semi-crystalline materials [[9]

and references therein]. The use of 3D structural methods

on C60 samples has allowed the clarification of the transfor-

mation mechanism from 2D rhombohedral C60 to DGLC

[111] and also identify new phases ‘‘compressed-graphite’’

formed during the C60 to diamond conversion [180]. Develop-

ment of pair-distribution-function computed tomography

allows the structure of nanocrystalline and amorphous mate-

rials to be identified, quantified and mapped, useful for mate-

rials such as catalysts [183,184]. In situ high-pressure XRD CT

and nanoCT is also actively being developed at synchrotron

sources and has been applied on carbon phases [185–187].

Details on other nanoprobes adapted for the study of Carbon

can be found in [185].

8.2. Open questions on C60 structural phase transitions

Based on the analysis of the current knowledge of structural

phase transition of C60, possible future directions for research

in this area include: (a) Establish a T–P protocol to prepare

‘pure’ single crystals which are necessary for the ultimate res-

olution of the structure and distortion of the different phases.

(b) Explore the structural organization, compressibility and

hardness of disordered phases derived from 2D/3D polymers,

considered as potential novel super-hard nanomaterials. (c)

Understand the stress-driven phase transformations and

the transformation mechanism between polymeric and disor-

dered phases, especially for 3D systems. (d) Enlarge the com-

prehension of 3D polymers as potential sp3 super-hard carbon

networks. (e) Model phase transitions, physical properties

and compressibility. (f) Validate the claimed hardness of the

novel phases. These are some of the open questions around

C60 that might be answered in the coming years. For comple-

mentary points of view the reader can refer to the next spe-

cialized reviews on C60 high-pressure transformations:

Moret [6], Nunez-Regueiro et al. (2000) [5], Sundqvist (1999)

[4] and Blank et al. (1998) [3].

Acknowledgements

M.A. acknowledges M. Monthioux and O. Degtyareva for

encouraging this review and for their support during the

preparation of the manuscript. Both authors acknowledge P.

Launois, A. San Miguel, L. Marques and M. Mezouar for com-

ments on the manuscript.

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