The factors influencing cation site-preferences in spinels a new mendelyevian approach

Abstract

Pseudopotential orbital radii rs, rp are used to construct an index, rσ=rs+rp, which characterizes the average potential experienced by atomic valence electrons. A plot of rAσverses rBσfor 172 chalogenide spinels (AB2X4, X=O, S, Se, Te) leads to two well defined areas, which separate normal and inverse spinels, with only four errors (a predictive success rate of 98%). The gross sorting is achieved without recourse either to the number of d-electrons or an orbital radius rd, from which it is inferred that it is the energies and extents of the cation s and p-orbitals which primarily determine coordination number in these systems. This approach to the problem of cation distribution in spinels is contrasted with the less generally applicable, traditional, crystal field ideas. The relevance of both rσ and crystal field stabilization energies to the thermodynamics of spinel reactions is also discussed.