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 B_Ge-O of GeO₆ groups denoted as [GeO₆], all bridging GeO₄ groups denoted as [GeO₄]_BO and GeO₄ groups with one non-bridging oxygen denoted as [GeO₄]^-_NBO have been determined for xNa₂O.(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 B_M-O of sodium germanate glasses has been suggested taking into account the contribution of different structural units namely [GeO₆], [GeO₄]_BO and [GeO₄]^-_NBO groups. It has been found that the average single bond strength B_M-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(n_o) 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(n_o) and B_M-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.