Article pubs.acs.org/JPCC
Combined First-Principles Calculations and Experimental Study of the Phonon Modes in the Multiferroic Compound GeV4S8 Elena Cannuccia,†,⊥ Vinh Ta Phuoc,‡ Benjamin Brière,‡ Laurent Cario,§ Etienne Janod,§ Benoît Corraze,§ and Marie Bernadette Lepetit*,∥,† †
Institut Laue Langevin, BP 156, 38042 Grenoble, France GREMAN CNRS UMR 7347, Université F. Rabelais, UFR Sciences, Parc de Grandmont, 37200 Tours, France § Institut des Matériaux Jean Rouxel, Université de Nantes, CNRS, BP32229, 44322 Nantes Cedex 3, France ∥ Institut Néel, CNRS UPR 2940, BP 166, 38042 Grenoble Cedex 9, France ‡
S Supporting Information *
ABSTRACT: 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 lowtemperature, ferroelectric phase (25K, Imm2), this is not the case for the high-temperature, paraelectric one within the F4̅3m group. Using group theory and first-principles calculations, we show that the IR/ Raman phonon modes are, however, compatible with the I4̅m2 space group. Analysis of the different modes at the ferroelectric transition shows that simultaneous weakening/strengthening of two symmetryrelated 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 F43̅ m group and the lattice dynamics I4m ̅ 2 group, can be interpreted within a dynamical Jahn−Teller distortion model for the paraelectric phase.
1. INTRODUCTION 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 ordering through the application of an electric field and of the polarization or dielectric constant through the application of a magnetic field. Multiferroic compounds are classified into type I and type II, according to whether the origin of the polarization is due to the inset of the magnetic ordering (type II) or to other mechanisms. Type I multiferroic compounds are characterized by a strong polarization but a weak magneto-electric coupling, while it is the reverse for the type II compounds. Recently a new class of type I multiferroic systems has been unveiled in which the polarization is induced by orbital ordering. Such a mechanism is extremely attractive because it theoretically allows us to have simultaneously large polarization and large magnetoelectric coupling. Indeed, we recently showed that GeV4S8, a member of the ternary chalcogenides family AM4Q8 (A = Ga, Ge; M = V, Nb, Ta; Q = S, Se), exhibits both a large polarization (∼0.75 μC/cm−1) and a strong sensitivity of the later to applied magnetic fields.1 In this compound the orbital ordering reorganizes both the charge and the spin within magnetic tetrahedral transition-metal clusters. These multiferroic properties may be expected to be more general in the family because most members present nonpolar to polar phases © 2017 American Chemical Society
transitions, and some of them were recently proved to present a magneto-electric coupling.2 The basic structure of the AM4Q8 compounds (see Figure 1) is built from the combination of AX4 tetrahedra and M4Q4 clusters, arranged in the NaCl manner. The M atoms form tetrahedral M4 clusters with typical intercluster distances of ∼4 Å and shorter M−M intracluster distances of
