Phase Equilibria in the Cu2S-Cu8SiS6-Cu8GeS6 System and Thermodynamic Functions of Phase Transitions of the Cu8Si(1−X)GeXS6 Argyrodite Phases

The phase equilibria of the Cu₂S-SiS₂-GeS₂ system have been studied in the Cu₂S-Cu₈SiS₆-Cu₈GeS₆ composition area. Based on data obtained from differential thermal analysis, powder x-ray diffraction, and SEM-EDS techniques, the T-x diagram of the Cu₈SiS₆-Cu₈GeS₆ boundary system and two internal polythermal sections, as well as the isothermal section at 300 K of the phase diagram and the liquidus surface of the Cu₂S-Cu₈SiS₆-Cu₈GeS₆ system were constructed. The areas of primary crystallization and homogeneity of phases, the nature, and temperatures of invariant and monovariant equilibria were determined. Continuous solid solutions based on both crystalline modifications of the starting compounds of the Cu₈SiS₆-Cu₈GeS₆ boundary system, have been revealed, which are of interest as environmentally friendly functional materials. The temperatures and enthalpies of phase transitions of Cu₈SiS₆ and Cu₈GeS₆ compounds, and Cu₈Si_(1−X)Ge_XS₆ solid solutions were determined using differential scanning calorimetry. The entropies of phase transitions for end-member compounds were also calculated. It is shown that the heats and entropies of phase transitions of these phases are anomalously large in comparison with the thermodynamic functions of ordinary polymorphic transitions. Apparently, this is due to a significant increase in the degree of disorder in the cationic sublattice upon transition to the high-temperature ion-conducting phase. It has also been established that the heats of phase transitions of solid solutions are practically equal to the sum of the corresponding functions of the end-member compounds.