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 SiO 2 -Al 2 O 3 -Na 2 O-P 2 O 5 had a metaluminous composition (Al:Na = 1) with the content of SiO 2 ranging from 50 to 70 mol% and of P 2 O 5 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 P 2 O 5 content (at the expense of NaAlO 2 ) and with increasing SiO 2 content (by substitution of SiO 4 for AlO 4 groups). This trend correlated with a decrease in density while, additionally, the formation of Al-O-P bonds with an SiO 2 -like structure may account for this change. The degree of ionic bonding, assessed via optical basicity and oxygen polarisability, decreased with increasing P 2 O 5 and SiO 2 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 P 2 O 5 and SiO 2 content. They indicated changing average bond strength of the glass network and showed a linear correlation with optical basicity. The average shift of high-frequency IR and Raman spectra, calculated as their centre of gravity (COG), correlates linearly with optical basicity and allows for estimating the average degree of covalent/ionic bonding by vibrational spectroscopy.