Antipolar transitions in GaNb4Se8 and GaTa4Se8

Applying dielectric spectroscopy and other techniques, the authors find antipolar ordering at the structural and magnetic transitions of the lacunar spinels GaNb${}₄$Se${}₈$ and GaTa${}₄$Se${}₈$, materials with substantial spin-orbit coupling. It arises from polar distortions of the moleculelike Nb${}₄$Se${}₄$ and Ta${}₄$Se${}₄$ clusters, typical for the crystalline lattice of lacunar spinels. Moreover, aside from dipolar relaxations and weak ferroelectric ordering, charge transport governed by in-gap impurity states is found at low temperatures, as previously suggested to explain the resistive switching in these compounds.