Raman spectroscopic study of structural changes in calcium aluminate (CaAl2O4) glass at high pressure and high temperature
The Raman spectrum of calcium aluminate (CaAl2O4) glass has been obtained as a function of pressure at ambient temperature to similar 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 Al species through the involvement of bridging oxygens (i.e. formation of 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 AlOAl 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 O2- ions from the fully polymerized aluminate framework, with the resulting formation at high temperature of O- species (''triclusters''), coordinated by three tetrahedral aluminate groups.