Cooperative cluster-Jahn-Teller effect as a possible route to antiferroelectricity

Abstract

We report the observation of an antipolar phase in cubic GaNb${}_4$S${}_8$ driven by an unconventional microscopic mechanism, the cooperative Jahn-Teller effect of Nb${}_4$S${}_4$ molecular clusters. The assignment of the antipolar nature is based on sudden changes in the crystal structure and a strong drop of the dielectric constant at $T_{\mathrm{JT}}=31$\,K, also indicating the first-order nature of the transition. In addition, we found that local symmetry lowering precedes long-range orbital ordering, implying the presence of a dynamic Jahn-Teller effect in the cubic phase above $T_{\mathrm{JT}}$. Based on the variety of structural polymorphs reported in lacunar spinels, also including ferroelectric phases, we argue that GaNb${}_4$S${}_8$ may be transformable to a ferroelectric state, which would further classify the observed antipolar phase as antiferrolectric.