Combined First-Principles Calculations and Experimental Study of the Phonon Modes in the Multiferroic Compound GeV4S8

The lattice dynamics of the GeV4S8 compound has been investigated using both density functional calculations and Raman/infrared (IR) measurements. While the accordance between the computed and the experimental data is very good in the low-temperature, ferroelectric phase (25K, Imm2), this is not the case for the high-temperature, paraelectric one within the F43m group. Using group theory and first-principles calculations, we show that the IR/Raman phonon modes are, however, compatible with the I4m2 space group. Analysis of the different modes at the ferroelectric transition shows that simultaneous weakening/strengthening of two symmetry-related bonds within the V4S4 cluster is a direct consequence of the orbital-order instability driving the ferroelectric transition. The softening of modes associated with such distortions call for strong orbital occupation fluctuations above Tc. These fluctuations, associated with the discrepancy between the X-ray scattering F43m group and the lattice dynamics I4m2 gr...