DFT study of the electronic properties and the cubic to tetragonal phase transition in RbCaF3

The structural, elastic, vibrational, and electronic properties of ${\mathrm{RbCaF}}_{3}$ in the cubic and low-temperature tetragonal phases have been studied at the ab initio level with density functional theory. Using various exchange-correlation functionals of the generalized gradient approximation for structural properties like the ${\mathrm{CaF}}_{6}$ octahedron rotational angle or ratio $c/a$ of the tetragonal lattice constants, it is found that the best agreement with experiment is obtained with the PBEsol and Wu and Cohen (WC) functionals. The fundamental band gap is calculated to be direct and indirect in the tetragonal and cubic phases, respectively. The relation between the cubic and tetragonal phases is studied by monitoring the cubic zone boundary soft mode phonon ${R}_{{15}^{\ensuremath{'}}}$ as well as the $c/a$ ratio and tilt angle. The results for the Born effective charge tensors are also reported in order to study the effect of the long-range Coulomb interactions. We also investigated the corresponding pressure driven phase transition at $T=0\phantom{\rule{4pt}{0ex}}\mathrm{K}$, which we observe to be of second order in contrast to the first-order character experimentally detected for the structurally similar high pressure transition at ambient temperature. Based on recently developed finite strain Landau theory, we offer a possible explanation for this peculiar change of character.