Abnormal refractivity trends in phosphate glass systems

Phosphate glasses differ from silicate glasses in the manner in which glass basicity (measured as optical basicity, Lambda) varies with increasing alkali or alkaline earth concentration. Whereas silicate glasses show a straightforward increase in Lambda, in the Na2O-P2O5 system, for example, there is almost no increase in A until the Na2O content exceeds approximately 50 mol%. This effect, previously observed by probe ion spectroscopy, is investigated here by studying trends in oxygen electronic polarisability (refractivity). These trends are explained by assuming that for phosphorus there are two basicity moderating parameters (gamma): for compositions more basic than metaphosphate gamma is 2.8, but for acidic glasses an additional gamma value of 2.1 must also be used owing to the presence of (-O-)(3)PO groups. Thus, the optical basicity of a glass in the range 0-50% basic oxide is calculated from Lambda = (x/gamma(M)+5x/2.8+5(100-2x)/2.1)/(500-4x) and for >50%, Lambda = (x/gamma(M)+5(100-x)/2.8)/(500-4x), where gamma(M) is the basicity moderating parameter of the basic oxide cation. The existence of these two gamma values provides scope for using refractivity data for structural investigations, this possibility is explored for the Al2O3-P2O5 and SiO2-P2O5 systems.