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We present dielectric, polarization, resistivity, specific heat, and magneticsusceptibility data on single crystals of the lacunar spinels GaNb4Se8 andGaTa4Se8, tetrahedral cluster-based materials with substantial spin-orbitcoupling. We concentrate on the possible occurrence of antipolar order in thesecompounds, as previously reported for the isoelectronic GaNb4S8, wherespin-orbit coupling plays a less important role. Our broadbanddielectric-spectroscopy investigations reveal clear anomalies of the intrinsicdielectric constant at the magneto-structural transitions in both systems thatare in accord with the expectations for antipolar transitions. A similaranomaly is also observed at the cubic-cubic transition of the Nb compoundleading to an intermediate phase. Similar to other polar and antipolar lacunarspinels, we find indications for dipolar relaxation dynamics at lowtemperatures. Polarization measurements on GaNb4Se8 reveal weak ferroelectricordering below the magneto-structural transition, either superimposed toantipolar order or emerging at structural domain walls. Thetemperature-dependent dc resistivity evidences essentially thermally-activatedcharge transport with different activation energies in the different phases. Ahuge step-like increase of the resistivity at the magneto-structural transitionof the Ta compound points to a fundamental change in the electronic structureor the mechanism of the charge transport. At low temperatures, charge transportis governed by in-gap impurity states, as also invoked to explain the resistiveswitching in these compounds.

Antipolar transitions in GaNb$_4$Se$_8$ and GaTa$_4$Se$_8$