Electronic structure and magnetic properties of cubic and hexagonal SrMnO3

SrMnO3 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 MnO6 octahedra only, the latter contains corner-sharing confacial bioctahedral Mn2O9 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 K but with quite different band gaps (0.3 vs 1.6 eV). The hexagonal ground state shows antiferromagnetic coupling both within the Mn2O9 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 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 SrMnO3 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 A 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 Mn2O9 dimer and facilitates the short Mn-Mn distance present in hexagonal SrMnO3. Hexagonal SrMnO3 is more compressible than cubic SrMnO3, owing to the more open structure. The calculated bulk modulus for hexagonal SrMnO3 is in good agreement with the high-pressure powder x-ray diffraction measurements also reported in the present paper.