Blue Spinel Crystals in the MgAl2O4-CoAl2O4 Series, Part I: Flux Growth and Chemical Characterization
Synthetic CoAl2O4 spinel, a high-temperature oxide (melting point of 1955°C), is the most stable compound of a family of spinel-structured oxides obtained from CO3O4 at progressive increase of Al contents. High-quality spinel single crystals with compositions closely corresponding to the solid-solution series spinel sensu stricto (MgAl2O4)-cobalt spinel (CoAl2O4) were produced by flux growth method, with Na2B4O7 as the flux. Low cooling rates and linear temperature profiles were applied in the thermal interval 1200–800°C, followed by rapid cooling. Thermal runs were performed in a reducing atmosphere. Selected crystals were investigated by SEM/EDS X-ray mapping to check for compositional homogeneity and by electron-microprobe analysis to obtain the chemical formula.
Crystals were found to be chemically homogeneous and entirely representing the MgAl2O4-CoAl2O4 solid-solution series, with the latter component ranging from 7% to 100%. The saturation of the vivid blue color increased with total cobalt content, but no shift in color hue was observed along the series. The unit-cell parameter a increased from 8.084 to 8.105 Å along the solid-solution series. The observed linear increase of the a parameter was seemingly due to the higher CoAl2O4 component. This was a premature and misleading conclusion, however, because structural changes in spinel may depend on both chemical composition and inversion parameter, which in turn is a function of thermal history. The composition of crystal products does not correspond to the composition of the starting oxide mixture: cobalt enriched in the crystals. A tentative explanation of this behavior is suggested by considering possible ionic potential as well crystal field stabilization effects.