A combined first-principles and experimental study of GeV4S8 compound
Multiferroicity and para/ferroelectric transition in GeV4S8. In the last years multiferroic compounds have attracted a lot of attention due to their coupled magnetic and electric properties. Indeed, when large, such a coupling allows the cross control of the magnetic field and of the polarization or dielectric constant through the application of a magnetic field. These systems also exhibit intrinsic lattice degrees of freedom leading to lattice instabilities and/or structural phase transitions. In this seminar I will report about the discovery of the multiferroicity in GeV4S8 compound and on the physical interpretation of paraelectric/ferroelectric transition occuring at 32 K [1,2]. [1] E. Cannuccia et al. J. Phys. Chem. C, 121, 3522 (2017) [2] K.Singh, et al. Phys. Rev. Lett. 113, 137602 (2014)
A combined first-principles and experimental study of GeV4S8 compound
Multiferroicity and para/ferroelectric transition in GeV4S8. In the last years multiferroic compounds have attracted a lot of attention due to their coupled magnetic and electric properties. Indeed, when large, such a coupling allows the cross control of the magnetic field and of the polarization or dielectric constant through the application of a magnetic field. These systems also exhibit intrinsic lattice degrees of freedom leading to lattice instabilities and/or structural phase transitions. In this seminar I will report about the discovery of the multiferroicity in GeV4S8 compound and on the physical interpretation of paraelectric/ferroelectric transition occuring at 32 K [1,2]. [1] E. Cannuccia et al. J. Phys. Chem. C, 121, 3522 (2017) [2] K.Singh, et al. Phys. Rev. Lett. 113, 137602 (2014)
![Conclusion
Ge
S
< 3 ˚A
≈ 4 ˚A
V
Elena Cannuccia (a), Kiran Singh(b), Charles Simon(a,b), Vinh Ta Phuoc (c), Benjamin Briere (c), Laurent Cario (d),
Etienne Janod (d), Benoît Corraze (d), and Marie Bernadette Lepetit (a,e)
a) Institut Laue Langevin, Grenoble France, b) Lab. CRISMAT, CNRS UMR 6508, ENSICAEN, 6 Bd. du Maréchal Juin, 14050 Caen, France
c) GREMAN, CNRS UMR 7347 Université F. Rabelais, Parc de Grandmont, 37200 Tours, France
d) Institut des Materiaux Jean Rouxel, Universite' de Nantes, CNRS, Nantes, France, e) Institut Neel, CNRS UPR 2940 Departement MCBT, Grenoble France
A combined first principle and experimental study of GeV4
S8
compound
GeV4
S8
is a new type I multiferroic compound
Molecularlike electronic states
are formed within the clusters.
Special class of Mott insulators because
of the possibility to tune the
insulatortometal transitions
Phase transitions in GeV4
S8
Searching for ferroelectricity
K.Singh, et al. Phys. Rev. Lett. 113, 137602 (2014)
Presence of finite polarization
The polarization increases
discontinuously:
paraelectric/ferroelectric
phase transition
The polarization decreases abruptly:
Magnetoelectric coupling
Lattice dynamics study to deep understand the para/ferro transition
IR
4 main modes:
309, 322, 370
and 445 cm1
50 K
zoom
445
370
309
322
2 softened modes
(445 and 322 cm1
)
below 50K
Theoretical phonon modes
and predicted IR intensities
E. Cannuccia, et al. JPC 121, 3522 (2017)
2 new modes
(333 and 342 cm1
)
visible at TS
Theoretical phonon modes
and predicted Raman intensities
0 new modes
visible at TS
300 K on a
single crystal
Detailed view of
low intensity region
50 K
zoom
E
B1
B2
E
E
E
B2
B2
A1
419
377
287
260
E. Cannuccia, et al. JPC 121, 3522 (2017)
Raman and Infra-red
measurement
8-300 K 22 – 300 K
✗ We predicted a metallic ground state:
WRONG
DOS
✗ Experimental and optimized geometry
significantly differ each other
✗ Global assignment of intense exper. modes:
NOT POSSIBLE
the B3LYP functional, we tried B1WC and PBE0 functionals
We questioned the spin-unpolarized calculation
the quality of the basis set, we tried with a plane-wave code
Having equivalent extinction
rules, a small sym. breaking is
hardly detected in X-ray
F43m I4m2
DOS
E. Cannuccia, et al. JPC 121, 3522 (2017)
We propose the assignement of the HT phase
phonon spectrum in the I4m2 group
Several phonon modes are softened below Ts,
and drive the intracluster atomic movements
observed in the ferroelectric transition
The simultaneous weakening/strengthening of VV
bonds is a direct consequence of the orbital order
instability seen as driving force of the
ferroelectric transition
Ge
S
Irrep E:
-200 cm-1
Physical interpretation of some phonon mode
Irrep B2
:
322 cm1
Softening of the mode below Ts
Bond/stretching
elongation V4
S4
cluster
Irrep E:
305 cm1
Any intracluster
movements
T [K]T [K]300
● cubic
● Assigned to F43m SG
● paraelectric
● Not centrosym. but
not polar
● insulating
3217
● orthorhombic
● Assigned to Imm2 SG
● ferroelectric
● Not centrosym, polar
● insulating
● orthorhombic
● Assigned to Pmn21
SG
● antiferromagnetic
● Polar, insulating
TN
TS
K.Singh, et al. Phys. Rev. Lett. 113, 137602 (2014)
Phonons in the ferroelectric (LT) phase Imm2
Orbital ordering Purely electric
polarization
Enhancement
of P
Structural
Transition
V1
V1
V2
V2
The origin of ferroelectricity in GeV4S8
Phonons in the paraelectric (HT) phase F43m
V1
V1
V2
V2
Any softening of the mode below Ts](https://image.slidesharecdn.com/postergev4s8-170626134310/85/a-combined-firstprinciples-and-experimental-study-of-gev4s8-compound-1-320.jpg?cb=1498484732)
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A combined first-principles and experimental study of GeV4S8 compound
- 1. Conclusion Ge S < 3 ˚A ≈ 4 ˚A V Elena Cannuccia (a), Kiran Singh(b), Charles Simon(a,b), Vinh Ta Phuoc (c), Benjamin Briere (c), Laurent Cario (d), Etienne Janod (d), Benoît Corraze (d), and Marie Bernadette Lepetit (a,e) a) Institut Laue Langevin, Grenoble France, b) Lab. CRISMAT, CNRS UMR 6508, ENSICAEN, 6 Bd. du Maréchal Juin, 14050 Caen, France c) GREMAN, CNRS UMR 7347 Université F. Rabelais, Parc de Grandmont, 37200 Tours, France d) Institut des Materiaux Jean Rouxel, Universite' de Nantes, CNRS, Nantes, France, e) Institut Neel, CNRS UPR 2940 Departement MCBT, Grenoble France A combined first principle and experimental study of GeV4 S8 compound GeV4 S8 is a new type I multiferroic compound Molecularlike electronic states are formed within the clusters. Special class of Mott insulators because of the possibility to tune the insulatortometal transitions Phase transitions in GeV4 S8 Searching for ferroelectricity K.Singh, et al. Phys. Rev. Lett. 113, 137602 (2014) Presence of finite polarization The polarization increases discontinuously: paraelectric/ferroelectric phase transition The polarization decreases abruptly: Magnetoelectric coupling Lattice dynamics study to deep understand the para/ferro transition IR 4 main modes: 309, 322, 370 and 445 cm1 50 K zoom 445 370 309 322 2 softened modes (445 and 322 cm1 ) below 50K Theoretical phonon modes and predicted IR intensities E. Cannuccia, et al. JPC 121, 3522 (2017) 2 new modes (333 and 342 cm1 ) visible at TS Theoretical phonon modes and predicted Raman intensities 0 new modes visible at TS 300 K on a single crystal Detailed view of low intensity region 50 K zoom E B1 B2 E E E B2 B2 A1 419 377 287 260 E. Cannuccia, et al. JPC 121, 3522 (2017) Raman and Infra-red measurement 8-300 K 22 – 300 K ✗ We predicted a metallic ground state: WRONG DOS ✗ Experimental and optimized geometry significantly differ each other ✗ Global assignment of intense exper. modes: NOT POSSIBLE the B3LYP functional, we tried B1WC and PBE0 functionals We questioned the spin-unpolarized calculation the quality of the basis set, we tried with a plane-wave code Having equivalent extinction rules, a small sym. breaking is hardly detected in X-ray F43m I4m2 DOS E. Cannuccia, et al. JPC 121, 3522 (2017) We propose the assignement of the HT phase phonon spectrum in the I4m2 group Several phonon modes are softened below Ts, and drive the intracluster atomic movements observed in the ferroelectric transition The simultaneous weakening/strengthening of VV bonds is a direct consequence of the orbital order instability seen as driving force of the ferroelectric transition Ge S Irrep E: -200 cm-1 Physical interpretation of some phonon mode Irrep B2 : 322 cm1 Softening of the mode below Ts Bond/stretching elongation V4 S4 cluster Irrep E: 305 cm1 Any intracluster movements T [K]T [K]300 ● cubic ● Assigned to F43m SG ● paraelectric ● Not centrosym. but not polar ● insulating 3217 ● orthorhombic ● Assigned to Imm2 SG ● ferroelectric ● Not centrosym, polar ● insulating ● orthorhombic ● Assigned to Pmn21 SG ● antiferromagnetic ● Polar, insulating TN TS K.Singh, et al. Phys. Rev. Lett. 113, 137602 (2014) Phonons in the ferroelectric (LT) phase Imm2 Orbital ordering Purely electric polarization Enhancement of P Structural Transition V1 V1 V2 V2 The origin of ferroelectricity in GeV4S8 Phonons in the paraelectric (HT) phase F43m V1 V1 V2 V2 Any softening of the mode below Ts