Corner- versus face-sharing octahedra in AMnO3 perovskites (A=Ca, Sr, and Ba)

The electronic structure of the series of perovskites AMnO(3) (A=Ca,Sr,Ba) is examined with the aid of density-functional calculations. A range of possible crystal structures is examined for each compound, and in each case the calculated lowest-energy structure is that observed at low temperature. The factors that control the variation in structure with the alkaline-earth ion A(2+) are discussed. CaMnO3 consists of corner-sharing octahedra but is orthorhombically distorted consistent with Ca2+ being too small for the 12-fold site within a perfect cubic MnO6 polyhedral framework. When the size of the alkaline-earth cation increases, a transformation from corner-sharing to face-sharing octahedra is induced since the alkaline-earth cation now becomes too large for the 12-fold site. While SrMnO3 at 0 K has the four-layered hexagonal (4H) structure with corner-sharing Mn2O9 dimers, BaMnO3 at 0 K adopts the two-layered hexagonal (2H) structure with infinite chains of face-sharing octahedra. The Mn charge is much lower than the conventional ionic model charge due to Mn-O covalence, and this reduces the Mn-Mn repulsion and favors sharing of the octahedral faces. We see no evidence for direct Mn-Mn metal bonding which has often been invoked to rationalize the adoption of this type of structure. We also discuss the atomistic origins of acid-base stabilization of ternary oxides from their binary constituents. A link between cation size and acid-base properties is suggested for AMnO(3).