Effect of Compression and Compositional Changes on the Nature of Chromium Sites in Simple Glasses

Abstract

The effects of pressure up to 50 kbar and of compositional changes on the absorption spectra of a number of chromium-containing silicate, phosphate, and borate glasses were observed. The spectra were analyzed into their component bands, and ligand-field theory was used to interpret the results to obtain information about the local symmetry, distortion, and local compressibility of the chromium sites. The absorption properties are explained by assuming that, on the average, the trivalent chromium ions are surrounded by six oxygen ions in an octahedral arrangement. The behavior of the spectra indicates that a random distribution of Cr–O distances about an average Cr–O distance exists and that only small disortions from cubic symmetry occur. The variation of the ligand-field parameters and the compressibilities of the sites depends mainly on the number of oxygen ions per unit volume in the glass, whether the changes are induced by external pressure or by compositional changes. Those glasses with a more open oxygen packing exhibit smaller crystal-field strengths, larger compressibilities, and larger distortions from octahedral symmetry than do the more densely packed glasses. Also, the compressibilities of the chromium sites are much less than the bulk compressibilities of the glasses.