Figure 1

Panel (a) shows the crystal structure of GaV₄S₈. The red V₄S₄ clusters and the green VS₄ clusters are arranged in an fcc lattice. Panel (b) illustrates the tetrahedral arrangement of the vanadium sites within the V₄S₄ clusters in the cubic phase of the crystal. The rhombohedrally distorted V₄ cluster is shown in panel (c). The distortion is exaggerated for visibility. The local electric polarization developing upon the phase transition along the remaining C₃ axis of the distorted tetrahedron is indicated by a red arrow. Panel (d) demonstrates the schematic setup for out-of-plane piezoresponse force microscopy (PFM), with the lateral x and y scanning directions indicated by red and green arrows, respectively. Panels (e–g) present optical micrographs of the three different GaV₄S₈ single crystals used in this study, i.e. #1 and #3 with (001) surfaces, and #2 exhibiting the (111) crystallographic surface. The typical dimensions of the cuboid-shaped crystals are approximately 1–2 mm. The high-symmetry directions within the as-grown surfaces are indicated with reference to the x and y PFM scanning directions.