1. Elena Cannuccia
Multiferroicity and
para/ferroelectric
transition in GeV4S8
04/04/2017
Lab. “Physique des Interactions Ioniques et Moleculaires”
Campus Saint Jerome, Marseille
Dipartimento di Fisica, Université de Rome “Tor Vergata”
Institut Laue Langevin, Grenoble

2. Ferromagnetism and Ferroelectricity
spin dipoles
Hz
Ez

3. Multiferroic Materials

4. Hi, I have strong
polarization and
weak magneto-
electric coupling
Hi, I have weak
polarization and
large magneto-
electric coupling
Type I Type II
Watch out for these two guys
(2 types of Multiferroic Materials)
P∼(1−100)μC /cm
2
P∼10−2
μC/cm2

5. Towards a new kind of
type I multiferroic
compounds
We are an assorted
group: charge and
orbital order MF
compunds are my
friends
I do not know
them !
Type I Type II
P=0.75μC /cm2

6. GeV4S8 is a new type
I multiferroic
compounds
Ge
S
< 3 ˚A
≈ 4 ˚A
V
Formation of molecular-like electronic
states within the clusters.
Special class of Mott insulators because of
the possibility to tune the insulator-to-metal transitions

7. T [K]T [K]
Phase transitions in GeV4S8
300
●
cubic
●
Assigned to F-43m SG
●
paraelectric
●
not centrosymm. but
not polar
●
insulating
3217
●
orthorhombic
●
Assigned to Imm2 SG
●
ferroelectric
●
Not centrosym, polar
●
insulating
●
orthorhombic
● Assigned to Pmn21
SG
●
antiferromagnetic
●
Polar, insulating
K.Singh, et al. Phys. Rev. Lett. 113, 137602 (2014)
TN
TS

8. The search of ferroelectricity
was then engaged
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:
Magneto-electric
coupling

9. The magneto-electric coupling
in GeV4S8
K.Singh, et al. Phys. Rev. Lett. 113, 137602 (2014)
The external magnetic field
affects the P magnitude and the
T dependence of P below TN
A deeper examination of the GeV4
S8
structure below TS
is
instructive in order to address the origin of the ferroelectricity.
GeV4
S8
is a very promising
multiferroic compound.

10. Orbital ordering Purely electric
polarization
Enhancement
of P
Structural
Transition
The origin of ferroelectricity
in GeV4S8
V1
-V1
V2
-V2
K.Singh, et al. Phys. Rev. Lett. 113, 137602 (2014)

11. Lattice dynamics study to deep
understand the para/ferro transitio

12. T [K]T [K]32
●
orthorhombic
●
Assigned to Imm2 SG
●
ferroelectric
●
polar
Raman and Infra-red
measurement
8-300 K 22 – 300 K
Lattice dynamics study to deep
understand the para/ferro transitio
Low Temperature phase High Temperature phase
●
cubic
●
Assigned to F-43m SG
●
paraelectric
●
not centrosymm. but not polar
●
insulating

13. Phonons in the ferroelectric (LT)
phase
IR
4 main modes:
309, 322, 370
and 445 cm-1
50 K
zoom
445
370
309
322
2 softened modes
(445 and 322 cm-1
)
below 50K
Theoretical phonon modes
and predicted IR intensities
E. Cannuccia, et al. JPC 121, 3522 (2017)
2 new modes
(333 and 342 cm-1
)
visible at TS

14. 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
Phonons in the ferroelectric (LT)
phase
E. Cannuccia, et al. JPC 121, 3522 (2017)

15. Phonons in the paraelectric (HT)
phase F43m
✗ 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 repeated with a plane-wave code

16. Let’s tackle the problem
from a more physical
point of view
Title:/srv/www/bcs_branch_www//bcs/www
Creator:dvips(k) 5.98 Copyright 2009 Rad
CreationDate:Mon Apr 3 11:27:25 2017Having equivalent extinction
rules, a small symm.
breaking is difficult
to detect in X-ray
F43m I4m2
DOS
E. Cannuccia, et al. JPC 121, 3522 (2017)

17. V1
-V1
V2
-V2
Ge
S
Irrep E:
-200 cm-1
Physical interpretation
of some phonon mode
Irrep B2
:
322 cm-1
Softening of the mode below Ts
Bond/stretching elongation V4S4 cluster
Any softening of the mode below Ts
Irrep E:
305 cm-1
Any intracluster movements
E. Cannuccia, et al. JPC 121, 3522 (2017)

18. Conclusions
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 V-V
bonds is a direct consequence of the orbital order
instability seen as driving force of the
ferroelectric transition

19. Marie Bernadette Lepetit
Laurent Cario Etienne Janod
Vinh Ta
Phuoc
And Kiran Singh, Charles Simon, Benoit Corraze, Benjamin Brière
Acknowledgements

20. Questions?