Electronic structure and magnetic properties of cubic and hexagonalSrMnO3

Abstract

$\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$ is a rare example of a compound having both a cubic (high-temperature) and a hexagonal (low-temperature) perovskite polymorph. While the former is built from corner-sharing $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra only, the latter contains corner-sharing confacial bioctahedral ${\mathrm{Mn}}_{2}{\mathrm{O}}_{9}$ entities along the $c$ axis. The electronic and magnetic structures of both polymorphs are investigated by density functional theory. Both the cubic and the hexagonal polymorphs are insulators at $0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ but with quite different band gaps (0.3 vs $1.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$). The hexagonal ground state shows antiferromagnetic coupling both within the ${\mathrm{Mn}}_{2}{\mathrm{O}}_{9}$ entities and between the Mn ions in the corner-sharing octahedra. The lowest energy cubic configuration is found to be $G$-type antiferromagnetic and is $260\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ per formula unit higher in energy than the hexagonal ground-state structure. While the bonding interactions involving Sr are found to be mainly ionic, there is a significant covalent contribution to the Mn-O bond. This covalency is very important for the stabilization of the hexagonal structure compared to the cubic polymorph. Two additional factors that stabilize hexagonal $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$ relative to the cubic polymorph are identified. (i) the Mn atoms in the face-sharing octahedra are displaced along the $c$ axis by about $0.012\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ from the center of the octahedra. (ii) The charge transfer giving lower charges for the oxygen in the face-sharing triangle compared to the corner-sharing oxygen. The latter effect results in a contraction of the oxygen triangle in the shared face. This negatively charged oxygen triangle effectively shields the repulsive interaction between the manganese atoms in the ${\mathrm{Mn}}_{2}{\mathrm{O}}_{9}$ dimer and facilitates the short Mn-Mn distance present in hexagonal $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$. Hexagonal $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$ is more compressible than cubic $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$, owing to the more open structure. The calculated bulk modulus for hexagonal $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$ is in good agreement with the high-pressure powder x-ray diffraction measurements also reported in the present paper.