Structure-property relationships in lacunar spinels from band theory*

The A-site deficient lacunar spinel GaM₄X₈ (M=Mo,V,Nb,Ta; X=S,Se) are ideal candidates to achieve novel electrical materials exhibiting metal-insulator transitions and may find use in resistive random-access memories (RRAM). They are experimentally narrow-bandwidth semiconductors, and undergo structural, electrical and magnetic phase transitions with external stimuli, e.g. temperature, pressure, electric pulse.

We systematically study the structure-property relationships within the lacunar spinels using ab initio density functional theory (DFT). We find that semi-local GGA functionals are able to predict insulating states with the low-symmetry rhombohedral phase without static correlation, but the high-temperature cubic phase is metallic due to orbital degeneracy. Spin-orbit coupling together with on-site Coulomb interactions are shown to reduce the local symmetry and open a small band gap within the cubic lattice. Our findings will assist understanding of the nature of these transitions and design new materials with tunable electronic states.