On the Structural and Electronic Factors Governing the Magnetic Properties of the Hexagonal Perovskite-Type Oxides AxBO3 (A=Ca, Sr, Ba; B=Co, Ni)

The hexagonal perovskite-type oxides AxBO3 (A=Ca, Sr, Ba; B=Co, Ni) consist of (BO3) infinity chains made up of face-sharing BO6 octahedra and BO6 trigonal prisms. On the basis of molecular orbital calculations for their structural building blocks we examined why the transition metal atoms in the trigonal prisms of Sr9/7NiO3 and Sr14/11CoO3 occupy the positions away from their trigonal prism centers and why Ba6/5NiO3 and Ca3/2CoO3 exhibit apparently puzzling magnetic properties. Our analysis indicates that the structural building units of the (BO3)infinity chains in these oxides adopt high-spin states to reduce the metal-metal sigma-antibonding interactions between adjacent metal atoms as well as the on-site repulsion. This finding led us to predict the number of unpaired spins that the oxides Sr9/7NiO3, Sr6/5CoO3, and Sr14/11CoO3 are expected to have.