An investigation of Al2O3 induced variations in the structural parameters in strontium borosilicate glasses using solid state NMR

Abstract

The physical properties of oxide glasses are crucially dependent on the atomistic structural speciation. In this study, we investigate the variation in the local ordering in the glass network of strontium borosilicate glasses (34.82 SrO, 51.84 B₂O₃, 13.34 SiO₂ in mol%) with a progressive substitution of B₂O₃ by Al₂O₃ and estimate the structural parameters: the oxygen packing fraction, and the average network coordination number. The coordination of the network forming cations at various glass compositions is determined using ¹¹B, ²⁷Al, and ²⁹Si solid-state nuclear magnetic resonance (SSNMR). The SSNMR reveals that at the higher substitution of B₂O₃ by Al₂O₃ in the glass composition, the coordination network of Al³⁺ exists predominantly in the 4 coordinated state, the network forming B³⁺ cations transform from a tetrahedral BO₄ to a trigonal BO₃ structure, and the Q⁴ form of silicates is dominant. The average coordination number and the oxygen packing fraction were calculated using the parameters obtained from the SSNMR results, and it is observed that the average coordination number decreases, and the oxygen packing fraction increases on incorporating Al. It is interesting to note that some of the thermophysical properties of these compositions closely follow the pattern shown by the average coordination number and the oxygen packing fraction.