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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 MgAl 2 O 4 -CuAl 2 O 4  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 Cu 2+ , 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 Cu 2+  contents. Short-range information derived from optical spectra suggests that the local TCu 2+ -O distances remain constant at increasing Cu 2+  content, whereas local MCu 2+ -O distances are ca. 0.02 Å shorter in Cu-poor MgAl 2 O 4  spinels as compared to MCu 2+ -O distances in end-member CuAl 2 O 4 . The observed splitting of an absorption band, at ca. 7000 cm -1  , caused by electron transitions in TCu 2+  as well as the anomalous broadness of an absorption band, at ca. 13 500 cm -1 , caused by electron transitions in MCu 2+  indicates the occurrence of local Jahn-Teller distortions at T and M. Long-range information, however, shows no violation of Fd3m symmetry. From refinements of our single-crystal XRD data we could for the first time derive for a cubic spinel phase a MCu 2+ -O distance of 2.080 Å and a TCu 2+ -O of 1.960 Å. The very limited variations in the unit-cell parameter a from 8.079 to 8.087 Å are mainly related to the disordering of Al. Because of the very similar size of Cu 2+  and Mg at the T and M sites, the spinel structure responds to the Cu 2+  → 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 MgAl 2 O 4 -CuAl 2 O 4  solid-solution series.

Cation ordering over short-range and long-range scales in the MgAl 2 O 4 -CuAl 2 O 4 series