Average single bond strength and optical basicity of Na2O- GeO2 glasses

Abstract

On the basis of the thermodynamical approach proposed by Takahashi and Yoshio the Ge-O single bond strengths BGe-O of GeO6 groups denoted as [GeO6], all bridging GeO4 groups denoted as [GeO 4]BO and GeO4 groups with one non-bridging oxygen denoted as [GeO4]-NBO have been determined for xNa 2O.(1-x)GeO2 (x = 5-30 mol%) glasses as follows: 323 kJ/mol for [GeO6], 341 kJ/mol for all bridging [GeO4]BO and 460 kJ/mol for [GeO4]-NBO groups. Equation for the calculation of average single bond strengths BM-O of sodium germanate glasses has been suggested taking into account the contribution of different structural units namely [GeO6], [GeO4]BO and [GeO 4]-NBO groups. It has been found that the average single bond strength BM-O of sodium germanate glasses decreases up to 267 kJ/mol at 25 mol% Na2O due to the formation of [GeO6] groups and increases up to 282.5 kJ/mol at 30 mol% Na2O due to the formation of [GeO4]-NBO groups. The results support the idea that the formation of [GeO6] groups is probably a main reason for the so-called germanate anomaly. The refractive index based optical basicity Λ(no) is sensitive in relation to the germanate anomaly effect and the group basicity follows the sequence: Λ[GeO4]BO < Λ[GeO6] < Λ[GeO4]-NBO. The observed correlation between Λ(no) and BM-O of sodium germanate glasses has been discussed with the presence of Ge(4)-O- Ge(4), Ge(4)-O-Ge(6) and Ge(4)-O-... Na(6) chemical bonds in the glass structure in which in bracket the coordination number of the cation is given. ©2009 The Ceramic Society of Japan.