Sol-gel syntheses and spectroscopic characterization of chromium-doped silicates and germanates

To avoid the technical difficulties encountered with high-temperature processing techniques, low-temperature sol-gel processes have been examined as routes to the Cr-doped olivine, Mg2SiO4, and the structural analogues Ca2SiO4 and Ca2GeO4. Gelation yields transparent, lightly colored gels that are converted to microcrystalline ceramic powders on heating to 850 degrees C. Initially doped with CrCl3.6H(2)O (0.01-1%), conversion to the ceramic is accompanied by changes in the coordination and oxidation state of the chromium. Consistent with size considerations, EPR, EXAFS, and XANES data indicate chromium exists principally as tetrahedrally coordinated Cr4+ in Ca2SiO4 and Ca2GeO4, and as a mixture of octahedrally coordinated Cr3+ and tetrahedrally coordinated Cr4+ in Cr-Mg2SiO4. Visible and near-IR electronic absorption and emission spectra reflect the composition of the material, and a comparison of the absorption of randomly oriented microcrystalline ceramics shows a surprising similarity in both band position and shape to polarized absorptions of forsterite single crystal