Phase Relations in the Ag2S-Ag8SiS6-Ag8SnS6 System and Thermodynamic Properties of Polymorphic Transitions of Argyrodite Phases

The Ag2S-Ag8SiS6-Ag8SnS6 system was studied using DTA/DSC, diffraction of X-ray, as well as SEM methods. The T-x phase diagram of the Ag8SiS6-Ag8SnS6 boundary system, several vertical and isothermal sections of the phase diagram, as well as a projection of the liquidus surface, were plotted, and the thermodynamic functions of polymorphic transitions of the Ag8SiS6 compound and Ag8Si1-xSnxS6 solid solutions were calculated. It was obtained that the Ag8SiS6-Ag8SnS6 boundary system is quasi-binary and is characterized by the formation of a continuous solid solutions during low-temperature orthorhombic and high-temperature cubic modifications of the initial compounds. It is shown that investigated system is a quasi-ternary plane of the Ag-Si-Sn-S concentration tetrahedron. The liquidus surface consists of two fields corresponding to the primary crystallization of high-temperature Ag8Si1-xSnxS6 solid solutions based on HT-Ag2S. Below the solidus, phase transformations associated with the polymorphism of the initial compounds and phases based on them were observed. Based on DSC data, the temperatures, enthalpies, and entropies of phase transitions of the argyrodite phases from low-temperature orthorhombic modification to high-temperature cubic modification were calculated. It was determined that the heats and entropies of tranformations of above phases have anomalously high values compared to ordinary polymorphic transitions. In addition, it was established that the entropies of phase transformations of solid solutions are practically equal to the sum of the corresponding functions of the original compounds. This indirectly indicates the quasi-ideality of solutions between both modifications of these compounds.