Crystal chemistry of silica-rich barium silicates

Identifying and explaining vibrational modes of sanbornite (low-BaSi2O5) and Ba5Si8O21: A joint experimental and theoretical study

We report here the analysis of vibrational properties of the sanbornite (low-BaSi₂O₅) and Ba₅Si₈O₂₁ using theoretical and experimental approaches, as well as results of high temperature experiments up to 1100-1150 °C. The crystal parameters derived from Rietveld refinement and calculations show excellent agreement, within 4%, while the absolute mean difference between the theoretical and experimental results for the IR and Raman vibrational frequencies was <6 cm^-1. The temperature-dependent Raman study renders that both sanbornite and Ba₅Si₈O₂₁ display specific Ba and Si sites and their BaO and SiO bonds. In the case of the stretching modes assigned to specific Si sites, the frequency dependence on the SiO bond length exhibited very strong correlations. Both phases showed that for a change of 0.01 Å, the vibrational mode shifted 10 ± 2 cm^-1. These results are promising for using Raman spectroscopy to track in situ reactions under a wide variety of conditions, especially during crystallization.

Nucleation and early stage crystallization in barium disilicate glass

Barium disilicate is one of the glass-ceramic systems where internal nucleation and crystallization can occur from quenched glass upon heat treatment without requiring nucleating agents. The structural origin of the nano-clusters formed during low temperature heat treatment is of great interest in gaining a fundamental understanding of nucleation kinetics in silicate glasses. Here, we present experimental investigations on the low temperature heat treatment of barium disilicate (BaO·2SiO₂) glass. Several experimental techniques were used to characterize the structural nature of barium disilicate glasses that were heat treated between the glass transition temperature, T_g, and the peak temperature of crystal growth, T_cr. The data show that small amounts of crystallites including BaSi₂O₅ as well as other higher Ba/Si ratio phases are formed. Moreover, unlike that reported for lower BaO content (BaO<33mol%) barium silicate glass or the analogous Li₂O-SiO₂ glasses, no clear evidence is observed for liquid/liquid phase separation in barium disilicate glass.