Présentation PowerPoint - GdR MCM-2...you will heard about that ! Diffraction of monochromatic X-rays by a single-crystal (halite) X-Rays Diffraction was discovered in 1912 by M

$Page 1: Présentation PowerPoint - GdR MCM-2...you will heard about that ! Diffraction of monochromatic X-rays by a single-crystal (halite) X-Rays Diffraction was discovered in 1912 by M$
11/18/2014

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Crystal-structure and micro-structure analysis to

investigate phase transitions

1

Pr Philippe GUIONNEAU

University of Bordeaux (France), ICMCB - CNRS

Photo-switch school, « structure determination » session

You are here now

Bird’s flight distance Paris-Bordeaux 500 km Bordeaux urban area 1 M hab.

ICMCB

Institute of Condensed Matter Chemistry of Bordeaux

http://www.icmcb-bordeaux.cnrs.fr/

CNRS – UPR9048

University of Bordeaux

http://www.u-bordeaux.fr/

60 000 students 5600 staff

70 laboratories 542 000 m2

528 M€ / year

Law and Economy Social and Human Sciences

Medicine Sciences and Technologies

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ICMCB Lead by Dr M. Maglione

250 people, 7 research teams

Solid-state Chemistry, Material Science, Molecular Science

4 research axes: Energy, Functional Materials, Nanomaterials, Sustainable dev.

ICMCB Team 6, “Switchable Molecules and Materials”

Created by Pr O. Kahn in 1995 – old name “Molecular Science Group”

Lead by Dr G. Chastanet since 2014

10 permanents

Chemistry, Physics and Theory of Molecular switches

ICMCB X-Ray Centre

Lead by Pr P. Guionneau

3 permanents

12 diffractometers (powder, single-crystal, film, capillary, raw material)

Optical microscopies, structural-data bases

4 – 1800 K, multi-wavelength, multi-gaz atmosphere

Routine to high-accuracy XRD data, texture and grazing incidence

From phase identification to ab initio structures

around 10 000 XRD data collection per year

Mechanism & nature of phase transitions

Structural-Phase Diagrams

Structure-properties relationships

Multi-scale knowledge of the switch

Molecular structures

and

Phase transition: event that induces a discontinuous change of at least one property

PHOTOSWITCH School:

Molecular Engineering and Investigation of Photoswitchable Systems

Crystallography thanks to X-Ray Diffraction (XRD):

- Lattice parameters - Symmetries - Atomic positions & motions - Molecular & Crystal structures - Disorder / defects - Dilatation / compressibility - Micro-structures

On single-crystals, powders, films or raw samples In variable temperature, pressure and light surroundings

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Back to basics

1-1 X-Ray Diffraction derived information

“What kind of information can you get from your XRD data ?”

1-2 Vocabulary about Phase Transitions

“Many different ways to name the same phenomenon ?”

Crystallography of a molecular switch: example of the Spin-CrossOver switch

Structural evidence for the molecular switch: structural movies

Mechanism(s) of the Phase Transition

Phase-diagrams (T, P, light)


Multi-scale description: from atoms to coherent domains


The visible light is diffracted by a small obstacle. X-Rays are diffracted by the atomic planes in crystals

It is basically the same phenomenon, it gives the same kind of images

Diffraction of a red light (laser) by a grid based on a few mm holes

Diffraction by light was discovered in 1665 by F.M.

Grimaldi but was first interpreted by Huygens in 1680

then by Fresnel and Fraunhofer based on Young’s

experiences of 1801

2015 is going to be the international year of light,

you will heard about that !

Diffraction of monochromatic X-rays by a single-crystal (halite)

X-Rays Diffraction was discovered in 1912 by M. von

Laüe and interpreted by the Bragg in 1913. This allowed

to prove that X-rays are electromagnetic waves and that

crystals are based on a periodic atomic structure.

It confirmed that matter is formed by atoms !

2014 is the international year of Crystallography

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X-Rays

X-Ray Diffraction

The position of Diffraction peaks is given by the Bragg’s law:

The measured intensities of Diffraction peaks is proportional to the square of structure Factors, that includes the Fourier transform of the electronic density in the crystal:

Measured intensity of the (hkl) Bragg peak:

Structure factor:

Atomic factor form of the (hkl) Bragg peak, Fourier Transform of the electronic density:

HKLd

n

2sin

Contains the unit-cell dimensions


The diffraction pattern shows the reciprocal space

Practical consequences of the Bragg’s law and the diffracted intensity formulae (1) :

“distances” are “inverted” in the diffraction image The smallest the crystal unit-cell sizes the longest the distances between diffraction peaks. If the unit-cell parameters increase then diffraction peaks become closer, and conversely. This is used to track phase transitions. The symmetries in the diffraction image are the same as in the crystal. The crystal system can be sometimes easily detected just by looking the diffraction images

Structural information can only be obtained if working on the FULL diffraction pattern. Working on a single diffraction peak will give you NO or only erroneous information.


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The inverse Fourier Transform of intensities gives the nature and positions of atoms in the asymmetric unit (thus in the unit cell) and their occupancy factors. It gives also the atomic displacement parameters (ellipsoid of probability of presence) that contains the information relative to thermal motion as well as defects for instance

The positions of the diffraction peaks give the unit-cell parameters. The crystal system is ultimately given by the space group determination.

Crystal symmetries – the space group – are derived from systematic absences of reflection (intensities that are equal to zero due to symmetrical reasons). For instance, some intensities may become equal to zero after a Phase transition, depending on the change of symmetry, or conversely they may appear in the new phase. That is used to study the mechanism of the phase transition.

The whole above parameters give the crystal structure

+

Monoclinic, P21/c

+

=



If the diffraction pattern is recorded for different or variable environments (T, P, light …) this gives access

to:

the phase diagrams through the determinations of all the crystal structures

the study of the phase-transition mechanism

the “movie” of the phase transition

compressibility and thermal expansion from the unit-cell parameters dependences

all information you need to draw structure-properties relationships

Observation of the entire diffraction image – “what happen between the Bragg peaks” –

give information on disorder or on any departure from a classical 3D periodic structure.


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1-Single-crystal, mosaïc, without defects

2-misalignement of domains

3-variation on unit-cell parameters

4-local defects

5-thermal motion

Disorder Scale Bragg peak profile Diffraction image

The examination of the diffraction-peaks shapes

give information at various scales, including the

morphology of coherent domains

Adapted from Snell et al. (2003), Macromolecular Crystallography,, 368, 268-288. 11

The ultimate pattern is a powder pattern

Information can also be retrieved from the shape of the Bragg peaks

The width of the peak is related to the size of the coherent domain


X-Ray Diffraction by powders or single-crystals ?

Large number (> 106 ) of very small crystals called crystallites

(< mm3), often agglomerated in particles. One particle may contain

many crystallites (coherent domain).

Powder

Unique atomic periodic network leading to a sample larger than a few mm. In practice a “mosaic” of slightly disorientated coherent domain

Single-Crystal

Powder X-Ray diffraction - PXRD Single-Crystal X-Ray diffraction - SCXRD

Superposition of the PXRD and SCRXD patterns

PXRD corresponds to a loss of spatial information in

comparison with SCXRD.


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Powder


Single-Crystal


Superposition of the PXRD and SCRXD patterns








Powder


Single-Crystal




But sometimes you have no SC. In addition, the study of powders

allows to investigate the crystallites and their microstructures.

Powders may exhibit different

structure-properties relationship than SC

Powders may resist better to phase-

transitions than SC

Powders are often closer from final applicative form than SC

So challenge PXRD !


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Intensity: atomic parameters

space group

Position: unit-cell parameters

Shape (width): microstructure

parameters

The background (in between the peaks):

parameters showing the

departure from the ideal 3D

periodic crystal –from atomic

(aperiodic packing …) to

microscopic and macroscopic

(twinning …) scales.

Scheme of the information retrieved from the Bragg peaks:

In fine, studying all these

aspects all along the SWITCH

gives you a fine description of

the associated

PHASE TRANSITION

Many softwares in many

environnments allow you to

determine these parameters:

Shelx’s family, Olex2,

Wingx,Topas, Fullprof, Winplotr,

SIR, Treor, Dicvol … etc …

It needs specific training

however to extract reliable

structural information



Phase transition (PT): event that induces a discontinuous change of at least one property (volume, density, elasticity, electric, magnetic, optical or chemical properties ….)

In the solid state, all phase transitions are accompanied by structural changes (sometimes very small)

Early nomenclature of phase transitions after Ehrenfest : Gibbs free energy, G= H-TS=U+pV-TS H: enthalpy; S: entropy; T: temperature; P: pressure; U: internal energy; V: volume

First-order transition: at least one of the first derivative of G shows a discontinuous change First derivatives of G are S and V DS 0 or DV 0; DH= TDS 0 (exchange of energy)

Second-order transition: at least one of the second derivatives of G shows a discontinuous change (V and S showing continuous changes) Second derivatives are the specific heat, Cp, the thermal expansion, aV, and the compressibility, KV, of the volume V

Accessible by XRD

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This definition is mainly based on macroscopic variables leading to ambiguous cases. A finest description is in used:

Discontinuous phase transition: entropy and an order parameter change discontinuously

the choice of the order parameter is crucial always accompanied by an exchange of latent heat, DH= TDS shows hysteresis shows metastable phases and co-existence of phases

Continuous phase transition: entropy and an order parameter change continuously

continuously means with “infinitesimal steps” no latent heat no hysteresis no co-existence of phases

Observable by XRD

An order parameter must: be 0 in the high-symmetry phase change continuously in the low-symmetry phase


A “structural” classification of phase transition (Buerger): Reconstructive phase transition: chemical bonds are broken and reconstructed and there is considerable atomic motions always “first order” the space group symmetries of the two phases are unrelated

Displacive phase transition: atoms experience small shifts

can correspond to a second or a first order phase transition the order parameter measures the 'distance' between the low- and high-symmetry phases not necessary accompanied by a change of space group

Order/disorder phase transition: the structural difference is related to an ordering of atoms/molecules / to different chemical occupation of the same crystallographic sites

A structural order parameter can be, for example: intensities (superstructure or absence conditions) atomic positions combination ex:(x-y)/(x+y) if x=y in the high-symmetry phase


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See for example « Symmetry Relationships between Crystal Structures” by U. Muller (2013)

The whole crystal does not transform at once. The phase transition may start in different zones of the sample. This nucleation can be simultaneous or not. Therefore, the phase transition may result in a domains structure. The sample can be seen as intergrown crystallites during the phase transition process. This is frequently encountered in molecular crystals. XRD investigation on the Bragg peaks shapes and distribution in the reciprocal space allow to describe the domains structure.

Further remark on the phase transition process when considering the crystal scale:

The relevant nomenclature of phase transitions, not unique, quite depends on the data you can access and on the purpose/context of your study.



Structural evidence for the molecular switch: structural movies

Mechanism(s) of the Phase Transition

Phase-diagrams (T, P, light)


Multi-scale description: from atoms to coherent domains

Fe2+ (d6)

T, P, h, B

Low Spin LS High Spin HS

1A1g 5T2g

strong crystal field weak crystal field

Property modifications : Magnetic

Optical (color) Structural (volume)

Potential applications: Electronic switches Information storage

Sensor X-chromic Pigment Molecular motors

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LS

Crosses: SQUID measurements

Circles: XRD structures

J. Phys. Chem. Solids, 73 (2012) 193

HS

0.5

T1/2

T(K)

HS

LS

Sequence of XRD molecular structures

21


Change of color

Change of magnetism Change of structure

Metal site

Molecule

Intermolecular

Crystal packing

Crystal

Microscopic

1

2

3

The structural information can be obtained from atomic to macroscopic scales

Structural films are obtained by a quasi-continuous crystal structure determination all along the SCO

100 150 200 250

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

ChiT

Ch

iT

Temperature (K)

100 150 200 2500,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

T (c

m3 K mol-

1)

T (K)

40 crystal-structures

HS

LS

S. Lakhloufi, PhD, ILL / ICMCB, 2013 22


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Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %

Molecule

Intermolecular

Crystal packing

Crystal

Microscopic

1

2

3

XRD structural film – example of [Fe(PM-AzA)2(NCS)2]

23


Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %

Molecule

Intermolecular

Crystal packing

Crystal

Microscopic

1

2

3 Histograms of Fe-N in FeN6 environment, search CSD

D<Fe-N>~0,20 Å

24


Low spin High spin

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Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %

Molecule

Intermolecular

Crystal packing

Crystal

Microscopic

1

2

3 Fe-N bond lengths = f(T)

D<Fe-N>= 0,20 Å

HS

LS

Sabine Lakhloufi, PhD, ILL/ICMCB, May 2013

25


Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %

Molecule

Intermolecular

Crystal packing

Crystal

Microscopic

1

2

3

HS

LS

Arnaud Grosjean, PhD, ICMCB, Dec 2013

Fe-N distances in 1D polymeric SCO

materials with hysteresis

P63/m

P63/m

Example of a SCO corresponding to a first-order

transition not accompanied with a space group change


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Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %

Molecule

Intermolecular

Crystal packing

Crystal

Microscopic

1

2

3

FeN6 volume HS

LS

Topics in Current Chemistry, 234, 97-128 (2004)

The same amplitude of DVpoly in all SCO materials

whatever the stimulus (T, P, light)

27


Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %

Molecule

Intermolecular

Crystal packing

Crystal

Microscopic

1

2

3

Angular distortion, S (): HS

LS

See for examples and ref. therein: Topics in Current Chemistry, 234, 97-128 (2004), Chem.Eur. J., 18, 5924-5934 (2012); Phys. Rev. B., 85, 064114 (2012)

=

6

1)(

i i NFeNFe

Also in use, length distortion (Å):

The distortion reflects the spin state and the SCO features

28


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Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %

Molecule

Intermolecular

Crystal packing

Crystal

Microscopic

1

2

3

Θ = )60(24

1

i i Trigonal distortion:

Trigonal prism

HS Regular octahedron

LS (Q =0°)

29


Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %

Molecule

Intermolecular

Crystal packing

Crystal

Microscopic

1

2

3

Θ = )60(24

1

i i Trigonal distortion:

The larger the distortion the higher T(LIESST)

30


Crystallographic evidence : Acta Cryst., 2005, B61, 25

see also M. A. Halcrow, Chem. Soc. Rev., 2011,40, 4119

see also P. Guionneau Dalton Trans., 2014, 43 (2), 382 – 393

Magnetic evidence: J.-F. Létard, J. Mater. Chem., 2006, 16, 2550

Theoretical evidence: S. Alvarez, J. Am. Chem. Soc., 2003, 125, 6795-6802

C. Boilleau et al. J. Chem. Phys., 2012, 137, 224304

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Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %

Molecule

Intermolecular

Crystal packing

Crystal

Microscopic

1

2

3

Example of no distortion in SCO materials:

Coordination polymer SCO materials [Fe(Rtrz)3]Ax·nH2O

S0° no angular distortion 0 Å no length distortion

D0° no trigonal distortion

NO distortion, NO LIESST effect

Eur. J. Inorg. Chem. (2013) 796 31


Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %

Molecule

Intermolecular

Crystal packing

Crystal

Microscopic

1

2

3

The larger the distortion the higher T(LIESST)

An EXTREME DISTORTION corresponds to

the HIGHEST photo-inscription temperature so far (T(LIESST) = 135 K)

Low Spin, FeN3C2O1

<Fe-N>= 1.935(3) Ǻ

Fe-O1= 2.240(2) Ǻ

Fe…O2= 3.200(2) Ǻ

T

High Spin, FeN3C2O2

<Fe-N>= 2.171(3) Ǻ

Fe-O= 2.334(2) Ǻ

Chem. Comm., 3723, (2007)

32

C2/c P21/c

The unique (so far) SCO associated with a change of

coordination number.

Example of SCO corresponding

to a reconstructive phase transition


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Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %

Molecule Deformation

Symmetry breaking

Ligand conformation

Intermolecular

Crystal packing

Crystal

Microscopic

1

2

3

HS LS

33


Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %


Symmetry breaking

Ligand conformation

Intermolecular

Crystal packing

Crystal

Microscopic

1

2

3

XRD structural film – example of [Fe(PM-AzA)2(NCS)2]

S. Lakhloufi, PhD ILL –ICMCB, 2013

34


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Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %


Symmetry breaking

Ligand conformation

Intermolecular Molecular interactions

Crystal packing

Crystal

Microscopic

1

2

3

Hirshfeld surface XRD structural film (f(T)) – example of [Fe(PM-AzA)2(NCS)2]

Hirshfeld surface – areas of intermolecular interactions

from XRD crystal structure

35


« Red zones witness an intermolecular distance inferior to the sum of Van der Waals radii . Modifications of these colors with T indicate

modifications of the intermolecular contacts “

Dalton Trans., 2014, 43 (2), 382 - 393

36

d(S…C)

Rai

deu

r d

e la

pen

te m

agn

étiq

ue

Example of the role of intermolecular interactions in SCO properties

Abruptness of the transition as a function of an intermolecular distance

The shortest S…C the more abrupt the SCO in the [Fe(PM-L)2(NCS)2] family

[Fe(PM-L)2(NCS)2]

abrupt

smooth

Topics in Current Chemistry, 234, 97-128 (2004), Phys. Rev. B., 85, 064114 (2012)

Magnetism, SQUID

Crystal Structure, SCXRD


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Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %


Symmetry breaking

Ligand conformation


Crystal packing Breathing

Dvunit-cell: 0 -10 %

Crystal

Microscopic

1

2

3

XRD structural film – breathing crystal packing 37


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Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %


Symmetry breaking

Ligand conformation




Crystal

Microscopic

1

2

3

Abrupt “respiration” due to a SCO corresponding to a first-order transition in a polymeric [Fe(NH2-trz)3].X2 single crystal

Eur. J. Inorg. Chem. (2013) 796

39


5 nm

15 nm

Respiration due to SCO in a polymeric [Fe(NH2-trz)3].X2 single crystal …

… the respiration is highly anisotropic and mainly along the chains.

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Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %


Symmetry breaking

Ligand conformation




Crystal

Microscopic

1

2

3

The « breathing » of the packing can also be followed by

the unit-cell dependence

a b

c

V


41


Getting the true “breathing” amplitude

due to the SCO only thanks to

isostructural non-SCO analogues

Chemical Physics Letters, 542 (2012) 52

100 120 140 160 180 200 220 240 260 280 3003550

3600

3650

3700

3750

3800

Un

it-c

ell

vo

lum

e (

A3)

Temperature (K)

[Fe(PM-AzA)2(NCS)2]

[Zn(PM-AzA)2(NCS)2]

J. Mater. Chem. (2002), 12, 2546 42


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[Zn(PM-AzA)2(NCS)2] [Fe(PM-AzA)2(NCS)2]

Packing modification in [300 – 100K]

SCO + thermal effects thermal effects

Chemical Physics Letters, 542 (2012) 52 43


The higher ai the value the easier the packing can be contracted in the direction.

The bulk modulus, aV-1 , reflects the ability of the unit cell to deform: the smaller the value the more

deformable is the cell. The tensor must be calculated from the unit-cell dependence outside the phase-transition region.

Example of dilatation-tensor calculation in the molecular complex [Fe(PM-PeA)2(NCS)2] .

The directions of main dilatation are strongly affected by the SCO in direction and amplitude.

aV-1 (3840 K at T=300K) increases by 10% at the HSLS SCO, showing a more deformable cell in HS

P. Guionneau et al., J. Mater. Chem., 12 (2002) 2546

a

c

1a

293 K

100 K

293 K

100 K

Using the cell parameters dependence to calculate the compressibility and dilatation tensors


SCO zone

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Using the cell parameters to calculate the compressibility and dilatation tensors

HS

LS

V

V

Dilatation tensors for the molecular SCO

complex [Fe(PM-AzA)2(NCS)2]

In this example, the packing is also much

more rigid in LS than in HS. There is also a

strong anisotropy of dilatation.

It tends to be a general result for SCO

materials.



(calculated from XRD data with the software PASCal,

Cliffe et al. J. App. Cryst., 2012, 45, 1321)

Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %


Symmetry breaking

Ligand conformation




Crystal

Microscopic

1

2

3


The SCO results in a rich variety of

structural phase transition

and therefore of

(P, T, light) phase diagrams

Dalton Trans., 2014, 43 (2), 382 - 393

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HS

Pbcn

T

Spin state

LS

Pbcn

P

hn

From laboratory XRD data: f(P, T, light).

The archetype for SCO molecular complexes


Packing symmetry

T. Granier et al., Inorg. Chem., 1993, 32, 5305 M. Marchivie et al., J. Am. Chem. Soc., 2002, 124, 194

[Fe(phen)2(NCS)2] (phen = 1,10-phenanthroline)

HS

Pccn

LS

P21/c

P

T T

HS

P21/c

Packing symmetry

Spin state

LS

Pccn P

hn hn

Chemical Physics (2013), 420, 25

Phys. Chem. Chem. Phys., 15, 13872 (2013)

Combination of DFT, molecular dynamic, XRD and NDR data.

Example of a synergy between

polymorphism and a SCO


[Fe(PM-BiA)2(NCS)2]

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49

Unit cell volume

Spin state

Pillet et al., PHYSICAL REVIEW B86, 064106 (2012)

HS

C2/c, V

LS

C2/c, V

2/3 LS

C2/c, 3V hn

hn

[Fe(bapbpy)(NCS)2]

Example of a SCO with

superstructure reflections

corresponding to a trebling of the

unit-cell volume


NH

N

NH

N

N

N

NH

N

NH

NNN FeFe

NCS

SCNSCN

NCS

HS

P21/n

LS

P21/n

Pre

ssure

T

½ LS

P-1

T hn

HS

P-1

50

HS

Cc

Packing symmetry

Spin state

Shepherd et al., Phys. Chem. Chem. Phys., 14, 5265-5271 (2012)

Example of a SCO with a

symmetry breaking in the

thermal- switch process inhibited

by pressure


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HS

P21/c, V

LS

Pccn, V

T, P

51

Packing symmetry

Spin state Example of a structural transition

associated with the SCO including

an unusual space group change –

the symmetry of the low-

temperature LS phase is higher

than the symmetry of the high-

temperature HS phase


J. Am. Chem. Soc., 199, 10861-10862, (1997) + (P) F. LeGac, PhD, ICMCB, Bordeaux, 2008

[Fe(PM-PeA)2(NCS)2]

100% HS 100% LS

Random process Continuous evolution of

Bragg peaks position

Domain Bragg peaks coexistence

Intermediate phase New diffraction pattern

52


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Random process Continuous evolution of

Bragg peaks position


Intermediate phase surstructure

HS HS/LS LS

224 K 240 K 218 K

J. Phys. : Condens. Matter, 19, (2007) 32611

Goujon et al., Phys. Rev. B, 2006, 73, 104413

Bürgi et al. , Angew. Chem. Int. Ed., 2003, 42:3825,

Törnroos et al., Chemistry A European Journal, 2006, 12, 24, 6207-6215

Pillet et al. Phys. Rev. B, 86, 064106 (2012)

53

Phys. Rev. B, 73 ( 2006) 060408



HS HS/LS LS

224 K 240 K 218 K

J. Phys. : Condens. Matter, 19, (2007) 32611 Phys. Rev. B, 73 ( 2006) 060408


The Bragg peaks modification with the constraints give information on the mechanism of the transition

T dependence of a relative diffraction peak intensity corresponding to the HS and LS phases.

It reflects the growth rate of the HS domains in the crystal

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T(K) T1/2↓ T1/2↑

HS HS

LS LS

Coherent domains co-existence inside the hysteresis loop


J. Phys. : Condens. Matter, 19, (2007) 32611

[Fe(PM-PeA)2(NCS)2]

Example of co-existence of HS and LS coherent domains during the SCO. The relative intensities give the conversion

rate (gHS)

HS LS

( at T1/2 : HS + LS)

Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %


Symmetry breaking

Ligand conformation




Crystal

Microscopic

1

2

3 Mosaicity, microstraints

Mechanism of transition

Coherent domains

Damages, Fatigability

Sample “respiration”

56

Particle (size is given by TEM)

Crystallites (single-crystals), the internal volume of each crystallite is the coherent domain (size is given by XRD)

The smaller the coherent domains the larger the Bragg peaks


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1- The Scherrer method P. Scherrer, Nachr. Ges. Wiss., 1918, 96.

bL: width of the Bragg peak due to coherent domain size

<L>: average apparent size

bmat= bL * bd

bobs= binst * bmat

You must know all terms or make (dangerous) approximations bL is an average size. It is also an “apparent” size. You must know the coherent domain morphology to get a “true” size.

bobs : observed experimentally binst: instrumental contribution bmat: material contribution bL: coherent domain sizes Bd: microstrains

but be careful, in fact:

give straightforward good results in a first approach

57


Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %


Symmetry breaking

Ligand conformation




Crystal

Microscopic

1

2

3 Mosaicity, microstraints


Coherent domains



58

170 nm

90 nm

40 nm

<L> ±1%

Grinding effects on the coherent domains size in a molecular

SCO material.


11/18/2014

30

Chem. Eur. J., 15, 6122-6130 (2009)

59

<L>

74 nm

145 nm

< 40 nm

Very old but unpublished results …


Playing with temperature to induce larger coherent domains and polymorphism. Easy observation with PXRD

Irreversible temperature-induced structural transition in a SCO molecular compound A(293 K)B (500K)C(293 K)

A B is a reconstructive phase transition B C ? Crystal structures are not solved yet

Chem. Eur. J., 15, 6122-6130 (2009)

TEM PXRD

Nb of coherent domains in 1 particle

Chem. Eur. J., 15, 6122-6130 (2009) 60


Comparison between TEM and XRD – particle and coherent domain – in a molecular SCO material

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31

1- The Scherrer method

Not easy to handle, time consuming, need high resolution PXRD data You need to know the exact crystal structure But it also gives you the MORPHOLOGY of the coherent domain

2- Analytical methods derived from the Scherrer methods

G. K. Williamson, W. H. Hall, Acta Metall. 1953, 1, 22.

B. E. Warren, B. L. Averbach, J. Appl. Phys. 1950, 21, 595.

V. Uvarov, I. Popov, Mater. Charact. 2007, 58, 883.

3- Rietveld refinement (profile matching)

Easy, first approach, some cautions to take however

4- Pair Distribution Function (PDF)

a specialist task ! Interesting for size < 5 nm


Difficult to handle on molecular materials

Comparison of coherent domain sizes calculated using

various methods.

Example on one molecular SCO powder sample.

Methods <L> (nm) Microstrains

Scherrer gaussien

100 Non available

Scherrer lorentzien

250 Non available

-

Williamson-Hall ~200

Probably strong

Profile matching 170 12 10-4

175 nm

62


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32

Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %


Symmetry breaking

Ligand conformation




Crystal

Microscopic

1

2

3

A. Grosjean, PhD, ICMCB, Univ. Bordeaux, 2013

Coherent domain size (nm) at 300 K as a function of the number

of SCO cycles from XRD powder diffraction Rietveld analysis .

In this case, results show a (relative) structural fatigability

Mosaicity, microstrains


Coherent domains



[Fe (Htrz)2(trz)](BF4)

Do

mai

n S

ize

(nm

) Number of complete LS-HS-LS thermal cycles

63


Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %


Symmetry breaking

Ligand conformation




Crystal

Microscopic

1

2

3

Chemical Physics Letters (2012) 52

What about single-crystals ? The mosaicity measures the slight misalignment of coherent domains in a single crystal. Measured from careful SCXRD and calculated from Bragg peaks widths.

[Fe(PM-AzA)2(NCS)2],

[Zn(PM-AzA)2(NCS)2 non SCO ( reference )

A slight structural fatigability in the first cycles, in this case

Mosaicity, microstrains


Coherent domains



64


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33

Metal site

<DFeN>: 0.2 Å

Distortion

DVpoly: 25 %


Symmetry breaking

Ligand conformation




Crystal

Microscopic

1

2

3 Mosaicity, microstrains


Coherent domains



65


The structural determination by XRD brings

much more than

the sole « molecular structure”.

The information is contained in your data.

It’s up to you to use it !

In the peculiar case of the SCO switch, it finally gives a multi-

scale view of a very complex phenomena showing a rich

diversity of structural behaviors.

Not everything is understood yet and probably just a little of

the richness has been discovered so far but one can already play

with this diversity to generate appropriate behaviors

Dalton Trans., 2014, 43 (2), 382 - 393

Co-workers in the studies presented today

ICMCB – Bordeaux - France

All the Gr6’s and specially:

Patrick Rosa

Mathieu Marchivie

Samir Matar

Guillaume Chastanet

Jean-François Létard

Cédric Desplanches

Arnaud Grosjean

Nathalie Daro

ICMCB X-Ray centre

Stanislav Pechev

Eric Lebraud

ILL- Grenoble - France

Marie-Hélène Lemée-Cailleau

Sabine Lakhloufi

LOMA – Bordeaux - France

Philippe Négrier

Denise Mondieig

LCC– Toulouse - France

Helena Shepherd

Gabor Molnar

Azzedine Bousseksou

Institut GeM-EMM- Nantes - France

Vincent Legrand

Diamond- Didcot - UK

Nicola Casati

And thanks to all co-authors of our SCO articles

Institut Jean Barriol - France

Sébastien Pillet

El-Eulmi Bendeif

Univ. Rennes - France

Eric Collet

Marylise Buron

Documents

Présentation PowerPoint - GdR MCM-2...you will heard about that ! Diffraction of monochromatic X-rays by a single-crystal (halite) X-Rays Diffraction was discovered in 1912 by M