Calculating Thermodynamic Properties of Alkali Borogermanates and Germanosilicates

Abstract

Evaluation of thermodynamic properties of the compounds, not studied experimentally, has been carried out with the use of regression analysis on the basis of the classical additive Neumann-Kopp method. Constitutive interdependence between the values of thermodynamic (thermochemical) potentials and the structures of substances (base substances) has been calculated according to the multiple regression equation. It has been necessary to introduce additional thermodynamic limitations caused by the properties of base components. The study of changes in enthalpies of formation in the series of borates, aluminates, fluoroaluminates, and arsenates of alkali metals, as well as aluminates and arsenates of alkaline-earth metals, has shown that the excess thermodynamic functions clearly depend on molecular masses of structural units of the same kind.

Composition dependencies of the standard entropy for lithium, sodium, and potassium silicates, borates, and germanates have been established; equations for the standard enthalpy of formation and heat capacity have been obtained. In order to refine the regression analysis the equations have been obtained with the help of weighting factors, which have been calculated in proportion to the fractions of germanate, silicate, and borate parts in Li₂O, Na₂O, and K₂O oxides, respectively. The summary table comparing the experimental and calculated values for enthalpies of formation of some potassium borogermanates shows that the disagreement between them is less than 5 %. The study results in calculated values of thermodynamic functions for alkali borogermanates and germanosilicates in the crystal state.