The influence of next nearest neighbours on cation radii in spinels: The example of the Co₃O₄-CoCr₂O₄ solid solution

Lattice parameters and crystal structures of the synthetic spinels Co₃O₄, CoCr₂O₄, and solid solutions in the binary join Co₃O₄-CoCr₂O₄, have been determined by powder X-ray diffraction structural refinements. In all these spinels the cation distribution is completely normal at room temperature, and the tetrahedrally coordinated cation site is occupied only by Co²⁺. The ionic radius of Co²⁺(tet) increases from 0.556(3) in Co₃O₄ to 0.599(4) in CoCr₂O₄. In the spinel structure, the interatomic distance between the tetrahedral cations and oxygen are geometrically independent of those between the octahedral cations and oxygen; thus the variation in effective ionic radii is ascribed to next-nearest neighbour effects, induced by covalent tendencies in the low-spin Co³⁺-O bond. The results demonstrate that the assumption of constant ionic radii even within an isomorphic group such as the oxide spinels needs to be made with caution.