Coupled Experimental Study and Thermodynamic Modeling of GeO₂ and GeO₂–CaO and GeO₂–SiO₂ Binary Systems

The phase diagram of the GeO₂–CaO system was investigated using differential thermal–thermogravimetric analysis and the equilibration–quenching technique, followed by electron probe microanalysis and X-ray diffraction. The Ca₂Ge₇O₁₆ compound was confirmed to be stable, and a eutectic reaction of L → GeO₂(hex) + Ca₂Ge₇O₁₆ was identified in the GeO₂-rich region. Based on the previous and present experimental data for the GeO₂–CaO system and reliable literature data for the GeO₂–SiO₂ system, thermodynamic optimization of both binaries was carried out. The GeO₂–SiO₂ system was reoptimized to ensure internal consistency and improved agreement with experimental data relative to the previous assessment, based on refinements to the heat capacity of GeO₂ and the quartz solid-solution model. The liquid phase was modeled using the Modified Quasichemical Model, and the quartz solid solution in the GeO₂–SiO₂ system, in which Si⁴⁺ and Ge⁴⁺ cations are mixed over a single sublattice, was described using the Compound Energy Formalism. A single set of self-consistent Gibbs energy functions for all phases in the GeO₂–CaO and GeO₂–SiO₂ systems was obtained, reproducing phase equilibria and thermodynamic properties over wide composition and temperature ranges. The optimized database provides a reliable foundation for thermodynamic modeling of GeO₂–CaO–SiO₂ ternary and multicomponent oxide systems relevant to metallurgical recycling as well as ceramic-glass design and processing.