Cation ordering over short-range and long-range scales in the MgAl2O4-CuAl2O4 series

A multi-analytical approach using electron microprobe analysis, X-ray structural refinement, and optical absorption spectroscopy was applied to characterize short-range and long-range structures of synthetic spinel single crystals along the MgAl2O4-CuAl2O4 solid-solution series. Site populations, derived from the results of site-scattering refinement and stereochemical analysis, show that the tetrahedrally coordinated site (T) is mainly populated by Mg and Cu2+, while the octahedrally coordinated site (M) is dominated by Al. Crystals also show a significant degree of inversion, i.e., occurrence of Al at T counterbalanced by occurrence of divalent cations at M, which increases slightly from 0.24 to 0.29 for the highest Cu2+ contents. Short-range information derived from optical spectra suggests that the local Cu-T(2+)-O distances remain constant at increasing Cu2+ content, whereas local Cu-M(2+)-O distances are ca. 0.02 angstrom shorter in Cu-poor MgAl2O4 spinels as compared to Cu-M(2+)-O distances in end-member CuAl2O4. The observed splitting of an absorption band, at ca. 7000 cm(-1), caused by electron transitions in Cu-T(2+) as well as the anomalous broadness of an absorption band, at ca. 13 500 cm(-1), caused by electron transitions in Cu-M(2+) indicates the occurrence of local Jahn-Teller distortions at T and M. Long-range information, however, shows no violation of Fd (3) over barm symmetry. From refinements of our single-crystal XRD data we could for the first time derive for a cubic spinel phase a Cu-M(2+)-O distance of 2.080 A and a Cu-T(2+)-O of 1.960 angstrom. The very limited variations in the unit-cell parameter a from 8.079 to 8.087 angstrom are mainly related to the disordering of Al. Because of the very similar size of Cu2+ and Mg at the T and M sites, the spinel structure responds to the Cu2+ -> Mg substitution by increasing cation disordering in such a manner that mean M-O distances remain constant and the mean T-O distances decrease slightly. This results in increasing length of shared octahedral edges and thereby increase of the octahedral cation-cation repulsion. In line with other studies, the importance of steric factors for controlling the cation distributions in the spinel structure is demonstrated to be valid also in the MgAl2O4-CuAl2O4 solid-solution series.