Solution Calorimetric Approach to the Structure of Alkali Germanate Glasses

Abstract

The heats of solution of alkali germanate glasses and crystals in the system R2O-GeO2 (R=Li, Na, K, Rb) were measured in 5% HF aqueous solution with an isothermal jacket calorimeter at 25°C. The relation between the heat of solution per mol of GeO2 and composition expressed by the mol ratio R2O/GeO2 was investigated and was discussed with respect to the structure and bond energy of the glasses and the corresponding crystals. The results are summarized as follows:1) The relation for glasses showed concave curvature in the composition range, R2O/GeO2<0.5 and showed linearity in the range, R2O/GeO2>0.5. The extrapolated values of the straight lines to the zero content of R2O correspond to the heat of solution of hexagonal crystalline GeO2, irrespective of alkali species.The above results suggest that the change, GeO4→GeO6, proceeds with increasing amount of R2O up to about the composition 0.33 R2O/GeO2, and the inverse change takes place by the further addition of R2O.These results for the glasses support the view of E. F. Riebling on structure of alkali germanate melts at 1300°C.2) For the crystals, the heat of solution increases linearly with increasing R2O content in the composition range from the pure GeO2 to the congruent melting compounds: Li2O⋅7GeO2, 2Na2O⋅9GeO2 and 3K2O⋅11GeO2 for respective systems. The slope of the lines becomes steeper in the R2O rich region beyond above compounds.3) The calculated heat of crystallization showed maximum at the composition of congruent melting compound in each R2O-GeO2 system.It is suggested from 1), 2) and 3) that the composition range in which GeO6 octahedra exist is more extensive in glasses than in crystals.4) From the heat of solution of glasses and the available thermodynamic data, the O-R bond energies in the glasses in the composition range, R2O/GeO2>0.5 were calculated on the basis of the method used on silicates in our previous work. The Ge-O bond energy in the glasses was also calculated as 82kcal/mol which is fairly smaller than 108 energy in the glasses was also calculated as 82kcal/mol which is fairly smaller than 108kcal/mol reported by K. H. Sun. The validity of the value obtained here was confirmed in comparison with the activation energy for viscous flow presented by previous investigators.