Cooperative Cluster Jahn-Teller Effect as a Possible Route to Antiferroelectricity

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