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A combined first-principles and experimental study of GeV4S8 compound

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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)

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A combined first-principles and experimental study of GeV4S8 compound

  1. 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