z Materials Science inc. Nanomaterials & Polymers
Order-Disorder Phase Transition, Anisotropic andSwitchable
Dielectric Constants Induced by Freeze of theWheel-Like Motion in a
Hexafluorosilicate-Based CrystalSijie Liu,[a, b] Jun Li,[b] Zhihua
Sun,*[a] Chengmin Ji,[a] Lina Li,[a] Sangen Zhao,[a] andJunhua
Luo*[a]
An order-disorder phase transition is found in a
hexa-fluorosilicate-based crystal, bis(betainium)
hexafluorosilicate bis(betaine) (1). At room temperature, 1
crystallizes in an ortho-rhombic space group Fddd and the
hexafluorosilicate moietyexhibits partial disorder which originates
from a uniaxial wheel-like rotation. As temperature decreases, a
total freeze of thiswheel-like motion results in a second-order
phase transition at250 K and the low temperature phase of 1 belongs
to a mono-clinic space group C/2c. Thermal measurements and
variable-
temperature single-crystal X-ray diffractions confirm that
dis-tinctive order-disorder transformation of the anionic rotor
isthe main driving force for the solid-state phase transition.
Fur-ther, theoretical computation of potential energies for
thewheel-like motion elucidates the dynamic changes in the
anionfrom rotating to static state. Moreover, dynamic changes of
thecrystalline in-plane rotor give rise to anisotropic and
switchabledielectric constants.
Introduction
Phase transition materials can display drastic changes in
elec-trical, magnetic, mechanical and optical properties at the
tran-sition temperature (Tc).
[1] Solid-state structural phase transitionhas played a crucial
role in imparting remarkable properties ofmany functional
materials, such as molecular ferroelectrics,nonlinear optical
switches, pyroelectric detectors and switch-able dielectrics,
etc.[2] Dynamic disorder in the local structurecan be used to
effectively design solid-state phase transitionmaterials.[3]
Usually, dynamic structural disorder is closely re-lated to various
types of interesting molecular or ionic motions.Order-disorder
transformation of ionic rotation or pendulummotion enables the
phase transition materials to be reversiblyswitched between two
states which are characterized by mark-edly different physical
properties.[4] For example, in a per-ovskite-type cage phase
transition compound (HIm)2[KCo(CN)6](HIm = imidazolium cation), the
structural disorder in the HImcation originates from its molecular
compass-like behaviour.Dynamic changes in the HIm cation from
static to rotating stateis the main driving force for the switching
between low andhigh dielectric states.[5] In addition, a unique
pendulum-like mo-
tion is found in a supramolecular complex potassium
hydrogenbis(dichloroacetate)-18-crown-6. Order-disorder
transformationof the pendulum-like motion in the dichloroacetate
anion trig-gers a switchable dielectric phase transition.[6]
As typical dynamically-disordered moieties, perchlorate,
tet-rafluoroborate and hexafluorophosphate moieties have beenused
to construct a variety of solid-state phase transition
mate-rials.[7] For instance, remarkable order-disorder
transformationof tumbling motion in the tetrafluoroborate moiety
helps to in-duce a ferroelectric phase transition.[8]
Hexafluorosilicate moietyhas also been used to assemble phase
transition materials. Forexample,
bis(tris(hydroxymethyl)methylammonium) hexa-fluorosilicate
undergoes a phase transition induced by a freez-ing of
reorientational motion of the hexafluorosilicate anion.[9]
And motions of the hexafluorosilicate moiety contribute to
themechanism of the first-order phase transition at around280/286 K
in phenylammonium hexafluorosilicate.[10] However,to the best of
our knowledge, only these two hexa-fluorosilicate-drived phase
transition materials were reported.In other
hexafluorosilicate-based phase transition materials,
thehexafluorosilicate anions are structurally ordered and the
phasetransitions are caused by cations or protons.[11] It is
com-paratively rare that hexafluorosilicate moiety dominates
thestructural changes during an order-disorder phase
transition.
In the present work, we found that the partial disorder ofthe
hexafluorosilicate moiety at room temperature originatesfrom its
disntinctive uniaxial wheel-like rotation in a
hexa-fluorosilicate-based crystal, bis(betainium)
hexafluorosilicate bis(betaine) (1)[12]. As temperature decreases,
a total freeze of thiswheel-like motion results in a second-order
phase transition at250 K. Thermal measurements and
variable-temperature single-crystal X-ray diffractions confirm the
solid-state phase tran-sition. Moreover, remarkable order-disorder
transformation of
[a] S. Liu, Prof. Z. Sun, C. Ji, Dr. L. Li, Dr. S. Zhao, Prof.
J. LuoKey Laboratory of Optoelectronic Materials Chemistry and
PhysicsFujian Institute of Research on the Structure of
MatterChinese Academy of SciencesFuzhou 350002 (China)E-mail:
[email protected]
[email protected][b] S. Liu, J. Li
University of Chinese Academy of SciencesChinese Academy of
SciencesBeijing 100039 (China)
Supporting information for this article is available on the WWW
underhttp://dx.doi.org/10.1002/slct.201601263
Full PapersDOI: 10.1002/slct.201601263
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http://dx.doi.org/10.1002/slct.201601263
the wheel-like motion gives rise to the anisotropic and
switch-able dielectric constants. Further, theoretical computation
ofpotential energies for the wheel-like motion elucidates the
dy-namic changes in the anion from rotating to static state.
Thiswork will be an advance toward understanding the
order-dis-order phase transition and the switchable dielectric
behavior.
Results and Discussion
Differential scanning calorimetry (DSC) and specific heat
ca-pacity (Cp) measurements are effective thermodynamic meth-ods to
investigate the solid-state phase transition of a com-pound. When
an order-disorder transition of ions or protons inthe compound
occurs, accompanied by thermal entropychange (DS), heat anomalies
can be detected in DSC and Cpmeasurements.[13] Thermal gravimetric
analysis (TGA) and differ-ential thermal analysis (DTA) show that 1
is thermally stable to513 K at which point the compound starts to
decompose (Fig-ure S3, Supporting Information). Then, the DSC
measurementof 1 was performed at the temperature range of 100–500
K. Atthis temperature range, DSC curves of 1 only exhibit a pair
ofbroad peaks around 250 K with a small thermal hysteresis,
in-dicating a reversible second-order phase transition (Figure
1).
Therefore, 1 only undergoes one phase transition at 250 K,
andthere is a large temperature gap (� 263 K) between
phasetransition and decomposition point. Cp measurement
furtherconfirms this solid-state phase transition and the typical
lshape of the heat anomaly most likely resembles the feature ofa
second-order phase transition, similar to that of TGS (trigly-cine
sulfate).[3] The integration of the l-shaped anomaly yieldedDS =
12.1 Jmol�1K�1. According to Boltzmann equation, DS =RlnN, where N
denotes the ratio of possible configurations andR is the gas
constant, it is found that N = 4.3, revealing that or-der-disorder
transition of ions or protons in 1 occurs. However,the deuterated
compound shows similar thermal behaviours to1 without any obvious
isotope effects, which would exclude
the possibility that this structural phase transition involves
pro-ton dynamics (Figure S4, Supporting Information). That is
tosay, the order-disorder structural transformation in 1
originatesfrom the dynamic motions of the ions, or to be more
exact, thehexafluorosilicate anions (please see the following
structuralanalysis).
Thermal measurements have confirmed that 1
undergoesorder-disorder structural phase transition at 250 K.
However,the origin of this structural change is unclear. To further
inves-tigate the order-disorder phase transition in 1, its crystal
struc-tures were determined at 293 K and 100 K (Table S1,
Support-ing Information). Although the disordered crystal structure
of 1at room temperature has been reported in the literature,[12]
wereanalyze the room-temperature structure and propose thatpartial
disorder in the hexafluorosilicate moiety originates fromits
uniaxial wheel-like rotation (please see the following struc-tural
analysis). The structure of room temperature phase (RTP,293 K) is
orthorhombic with a centrosymmetric space group ofFddd, and cell
parameters of a = 13.2237, b = 19.3136, c =22.0443 �, a =b= g= 908,
and V = 5630.05 �3. These structuralinformation accords with the
crystallographic data reported inthe literature. However, the
structure of low temperature phase(LTP, 100 K) differs greatly from
that of RTP and has never beenreported before. At LTP, 1
crystallizes in the monoclinic systemwith a centrosymmetric space
group of C2/c, and the cell pa-rameters turns to a = 13.022, b =
21.9525, c = 11.4831 �, a=g=908, b= 123.5238 and V = 2736.6 �3. The
solid-state crystallinetransformation of 1 can be depicted with an
Aizu notation ofmmmF2/m, suggesting a potential ferroelastic phase
tran-sition.[14] A symmetry breaking phenomenon occurs during
thephase transition, namely, the total symmetry decreases by
halffrom eight symmetric elements (E, C2, 2C’2, i, sh, 2sv) to four
(E,C2, i, sh). According to Landau phase transition theory,
thegroup-subgroup relationship and halving of the symmetric
ele-ments indicate that 1 undergoes a second-order phase
tran-sition, which corresponds well to the thermal properties.
The basic structural unit of 1 consists of one
hexa-fluorosilicate anion and two dimeric (betaine-H-betaine)+
cati-ons (Figure 2). All the atoms of the cations are determined
atexclusive positions at both LTP and RTP, showing no
obviousdisordering. The dotted pink lines represent the strong
inter-molecular hydrogen bonding interactions. During the
phasetransition, the slight difference in the hydrogen bonding
inter-actions is that the distance of O�H···O hydrogen bond
changesfrom 2.429 to 2.432 �. This small variation indicates that
protondynamic probably is unrelated to the phase transition of
1,which agrees well with the above-mentioned thermal analysis.It is
remarkable that the hexafluorosilicate moiety exhibits dis-tinctive
order-disorder transformation in response to temper-ature change.
At RTP, the hexafluorosilicate anion is partiallydisordered,
namely, the Si and the two polar F atoms are or-dered; while the
other four equatorial F atoms are strongly dis-ordered. The 1808
F�Si-F bonds through the two polar F atomsact as a fixed rotational
axis, as shown by the arrowhead. Strik-ingly, the four equatorial
F�Si bonds rotate around this axis,forming a dynamic wheel-like
architecture. In other words, thehexafluorosilicate group in 1
behaves like an in-plane rotor and
Figure 1. Differential scanning calorimetry (DSC) and specific
heat capacity(Cp) measurements reveal a reversible second-order
solid-state phase tran-sition in 1. DSC curves obtained in a
cooling-heating cycle, and Cp curve ob-tained in the heating
mode.
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changes in the anion from rotating to static state makes a
sig-nificant contribution to the phase transition in 1.
Usually, phase transition is accompanied by an anomaly
ofphysical property (e. g. dielectric constant, e’) at Tc. The
highly-disordered state induced by molecular motions would
corre-spond to the high dielectric state. During the phase
transition,molecular motions are frozen on cooling and cause a
decreasein e’. Therefore, temperature-dependent e’ would provide
rele-vant information on dynamics of phase transitions.[2c, 4c, 5,
13c, 16]
Here, variable-temperature dielectric spectroscopy of
singlecrystals of 1 was measured at a frequency of 100 KHz (Figure
6).
The crystal faces are selected in accordance with the
structurein the RTP. A strong crystal axis-dependence of e’ in 1 is
found.For compound 1, the wheel-like motion at room temperature
is
confined in the equatorial plane (ab-plane), therefore in the
di-rections parallel to the equatorial plane (e. g. // b), e’ is
large (�10.9). However, in the directions parallel to rotational
axis (i. e.// c), the F�Si-F bonds are fixed and e’ is relatively
small (�10.0). Upon cooling, obvious anomaly (De’ � 0.9) is
observedin the directions parallel to the equatorial plane (e. g.
// b)around Tc, because the the wheel-like motion is frozen. In
con-trast, very small dielectric anomaly (De’ � 0.1) is recorded
atthe whole temperature range in the direction of the fixed
rota-tional axis (i. e. // c).Therefore, distinctive order-disorder
trans-formation of the wheel-like motion gives rise to the
anisotropicand switchable dielectric constants in 1.
Conclusions
In summary, an order-disorder phase transition is found in
ahexafluorosilicate-based crystal, bis(betainium)
hexa-fluorosilicate bis(betaine). At room temperature, the
hexa-fluorosilicate moiety behaves like a in-plane rotor and
displaysa distinctive uniaxial wheel-like rotation. Upon cooling,
thefreeze of the wheel-like motion gives rise to the
second-ordersolid-state phase transition in 1. Variable-temperature
single-crystal X-ray diffractions, dielectric spectroscopy, thermal
meas-urements and theoretical computation confirm that
dynamicchanges in the crystalline rotor dominate the phase
transitionbehaviour. This investigation will contribute to a deeper
under-standing of the order-disorder phase transition and the
switch-able dielectric constants.
Supporting information
Experimental Section, Figure S1-S7, Table S1 and X-ray
crystallo-graphic information files of 1 in different phases, CCDC
refer-ence numbers 1449144 and 1449145.
Acknowledgements
This work was supported by NSFC (21525104, 91422301,21373220,
51402296, 21571178, 51502288, 51502290 and21301172), the NSF for
Distinguished Young Scholars of FujianProvince (2014 J06015 and
2016 J06012), the NSF of Fujian Prov-ince (2014 J01067, 2014 J05068
and 2015 J05040), Youth In-novation Promotion of CAS (2014262,
2015240 and 2016274),and the supports from “Chunmiao Projects” of
Haixi Institute ofChinese Academy of Sciences (CMZX-2013-002 and
CMZX-2014-003). Z.S. and S.Z. thank the supports from the “Team
HundredTalents Plan” of Haixi Institute of Chinese Academy of
Sciences.We acknowledge the Supercomputing Center of CNIC for
provid-ing the computer resources.
Keywords: Anisotropic and switchable dielectric constants
·hexafluorosilicate anion · molecular dynamics · phasetransitions
solid-state structures
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Submitted: September 7, 2016
Accepted: September 27, 2016
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