• journal_title：American Mineralogist
• Contributor：Rosa Anna Fregola ; Ferdinando Bosi ; Henrik Skogby ; Ulf Hålenius
• Publisher：Mineralogical Society of America
• Date：2012-11-01
• Format：text/html
• Language：en
• Identifier：10.2138/am.2012.4177
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：97
• issue：11-12
• firstpage：1821
• section：Articles

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 ^TCu²⁺-O distances remain constant at increasing Cu²⁺ content, whereas local ^MCu²⁺-O distances are ca. 0.02 Å shorter in Cu-poor MgAl₂O₄ spinels as compared to ^MCu²⁺-O distances in end-member CuAl₂O₄. The observed splitting of an absorption band, at ca. 7000 cm⁻¹, caused by electron transitions in ^TCu²⁺ as well as the anomalous broadness of an absorption band, at ca. 13 500 cm⁻¹, caused by electron transitions in ^MCu²⁺ indicates the occurrence of local Jahn-Teller distortions at T and M. Long-range information, however, shows no violation of Fd3̄m symmetry. From refinements of our single-crystal XRD data we could for the first time derive for a cubic spinel phase a ^MCu²⁺-O distance of 2.080 Å and a ^TCu²⁺-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 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.