Cooperative Cluster Jahn-Teller Effect as a Possible Route to Antiferroelectricity
K. Geirhos, Jan Langmann, L. Prodan, Alexander A. Tsirlin, Aleksandr Missiul, Georg Eickerling, Anton Jesche, V. Tsurkan, P. Lunkenheimer, Wolfgang Scherer, I. Kézsmárki
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_{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_{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 antiferroelectric.