Dispersion, ionic bonding and vibrational shifts in phospho-aluminosilicate glasses

Understanding the relationships between structure and properties of aluminosilicate glasses is of interest in magmatic studies as well as for glass applications as mechanical or optical components. Glass properties may be tailored by the incorporation of additional elements, and here we studied the effect of phosphate incorporation on refractive index and the degree of ionic bonding in aluminosilicate glasses. The studied glasses in the system SiO₂-Al₂O₃-Na₂O-P₂O₅ had a metaluminous composition (Al:Na = 1) with the content of SiO₂ ranging from 50 to 70 mol% and of P₂O₅ from 0 to 7.5 mol%. Refractive index was measured at four wavelengths from visible to near-infrared and found to decrease both with increasing P₂O₅ content (at the expense of NaAlO₂) and with increasing SiO₂ content (by substitution of SiO₄ for AlO₄ groups). This trend correlated with a decrease in density while, additionally, the formation of Al-O-P bonds with an SiO₂-like structure may account for this change. The degree of ionic bonding, assessed via optical basicity and oxygen polarisability, decreased with increasing P₂O₅ and SiO₂ content. Despite the complexity of the studied glasses, oxygen polarisability and optical basicity were found to follow Duffy's empirical equation for simple oxide glasses. In the high frequency infrared and Raman spectra, band shifts were observed with increasing P₂O₅ and SiO₂ content. They indicated changing average bond strength of the glass network and showed a linear correlation with optical basicity.