Cation distribution and bond lengths in CoAl2O4 spinel

It is widely known that CoAl2O4, IV(Co1−xAlx) VI[CoxAl2−x]O4, is a largely normal spinel (0<x<2/3), but this cation distribution cannot be explained by the effects of cation size, electronegativity and ligand-field of Co2+. To examine the reason for this peculiar cation distribution, the relationship between the inversion parameters (x) and the interatomic distances was investigated using the Rietveld analysis of X-ray diffraction. On the basis of the results of the structure refinements by Rietveld analysis, the regression equations for the mean bond lengths and O⋯O distances as a function of x were calculated by a linear least-squares method. The variations of local bond lengths versus x were inferred from the regression equations for the mean bond lengths. With increasing x, the ratio (O⋯O)shared/(O⋯O)unshared in VIB-site octahedron increases remarkably and the local bonds on IVA-site are inferred to be abnormally lengthened. These results imply that if CoAl2O4 were a largely inverse spinel, the local bond lengths on IVA-site would become abnormally long and the repulsion between VIB-site cations would become considerably large. Such destabilization factors of the crystal structure can be avoided by the largely normal configuration of cations.