On the structural and electronic factors governing the magnetic propertiesof the hexagonal perovskite-type oxides A(x)BO(3) (A = Ca, Sr, Ba; B = Co,Ni)

The hexagonal perovskite-type oxides A(x)BO(3) (A = Ca, Sr, Ba; B = Co, Ni) consist of (BO3)(infinity) chains made up of face-sharing BO6 octahedra an d BO6 trigonal prisms. On the basis of molecular orbital calculations for t heir structural building blocks we examined why the transition metal atoms in the trigonal prisms of Sr9/7NiO3 and Sr-14/11 CoO3 occupy the positions away from their trigonal prism centers and why Ba6/5NiO3 and Ca3/2CoO3 exhi bit apparently puzzling magnetic properties. Our analysis indicates that th e structural building units of the (BO3)(infinity) chains in these oxides a dopt high-spin states to reduce the metal-metal a-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 Sr-14/11 CoO3 are expected to have. (C) 2001 Academic Press.