Behavior of Chromium in the System MgA1 2 O 4 – A1 2 O 3

Single crystals of chromium‐doped magnesium aluminate spinels with varying amounts of excess alumina have been grown by the flame fusion process. Presumably these crystals contain stoichiometrically significant amounts of defects. When these chromium‐doped spinels are annealed at temperatures below their melting point, the spinel structure cannot tolerate such a high concentration of defects and at least some of the excess alumina is exsolved. The chromium ion in magnesium aluminate spinel is pink, whereas in magnesium aluminate spinel containing excess alumina it is green. When exsolution of the alumina occurs during annealing, the aggregate changes to pink. The behavior of the chromium ion is monitored by observing the significant changes in the optical absorption as well as the emission or fluorescence spectra for both single crystals and polycrystalline materials in the system [MgA1 2 O 4 – A1 2 O 3 ] :Cr.