CRUCIAL INTERPLAY OF ORBITAL AND CATION ANION INTERACTIONS IN THE SOLID-STATE - DISTORTIONS OF NIO6 OCTAHEDRA IN BANILN2O5 OXIDES

The diverse structural features of the three families of oxides belonging to the stoichiometry BaMLn2O5 (M = Co, Ni, Cu, Zn, Pd, Pt; Ln = Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, or Y) are described. One of these families, typified by the BaNiGd2O5 structure, possesses a one-dimensional chain of vertex-sharing NiO6 octahedra running parallel to the crystallographic a axis but exhibits a distorted octahedral coordination about Ni11. The Ni-O contacts to the two oxygen ligands along the axis are considerably shorter than those to the four remaining oxygen ligands in the equatorial plane perpendicular to a (axial = 1.8936 (2) Å; equatorial = 2.197 (6) Å), a very unusual result. In addition, the equatorial oxygen ligands are distorted away from the right angles of an ideal octahedron to 79.6 (2)°, yielding a D2h local geometry about Ni. These observations are understandable using a model which combines results from molecular orbital theory, tight-binding band structure calculations, and empirical atom-atom potential arguments. © 1990, American Chemical Society. All rights reserved.