Modeling the structural distortion and magnetic ground state of the polar lacunar spinel GaV 4 Se 8

The lacunar spinel ${\mathrm{GaV}}_{4}{\mathrm{Se}}_{8}$ is a material whose properties are dominated by tetrahedral clusters of V atoms. The compound is known to undergo a polar distortion to a ground state structure in the $R3m$ space group, and orders ferromagnetically with a relatively small magnetic moment. We develop an understanding into the relationship between crystal structure and magnetic order in this material, and the influence of electron correlations in establishing the observed ground state using first-principles density functional theory (DFT) electronic structure calculations. Because electrons are delocalized within ${\mathrm{V}}_{4}$ clusters but localized between them, the usual approaches to simulate electron correlations---such as the use of the Hubbard $U$ in $\mathrm{DFT}+U$ schemes---do not adequately recreate the experimental ground state. We find instead that the experimental ground state of ${\mathrm{GaV}}_{4}{\mathrm{Se}}_{8}$ is well represented by the random-phase approximation to the correlation energy. Additionally, we find that magnetism and crystal structure are strongly coupled in this material, and only certain arrangements of magnetic moment within a ${\mathrm{V}}_{4}$ cluster can stabilize the observed structural distortion. In combination with the anisotropic, polar nature of the material, the strength of magnetostructural coupling indicates that application of strain could be used to tune the magnetic properties of ${\mathrm{GaV}}_{4}{\mathrm{Se}}_{8}$.