Raman spectroscopic study of structural changes in calcium aluminate (CaAl 2O 4) glass at high pressure and high temperature

Abstract

The Raman spectrum of calcium aluminate (CaAl 20 4) glass has been obtained as a function of pressure at ambient temperature to ∼ 15 GPa, and as a function of temperature at room pressure to 1650 K. On increasing pressure at 300 K, the Raman spectrum changes drastically between 8 and 10 GPa. The intensity of the overall spectrum weakens, the intensity of the 580-cm −1 band decreases markedly, and a new broad band centered near 750 cm −1 begins to dominate the spectrum. We interpret these spectral changes as due to the formation of highly coordinated A1 species through the involvement of bridging oxygens (i.e. formation of [3]O species). Upon decompression, the pressure-induced spectral changes are mostly reversible. However, the decompressed samples do not relax completely to their normal ambient state, and the spectra reflect important changes in the inter-tetrahedral AlOA1 linkages. These changes are consistent with some high-coordinate species being retained in the decompressed sample. On increasing temperature through the glass transformation range ( T g = 1178 K) at room pressure, our Raman spectra show evidence for important configurational changes, after vibrational effects are removed. The major spectral change consists of growth of a broad band between 600 and 1100 cm −1 appearing immediately above T g. We suggest that these configurational changes concern the removal of O 2− ions from the fully polymerized aluminate framework, with the resulting formation at high temperature of [3]O species (“triclusters”), coordinated by three tetrahedral aluminate groups.