Shender I. Effect of isovalent cation substitution on the physical properties of Ag7(Si1–xGex)S5I superionic conductors.

For the first time, single crystal samples have been obtained, and superionic ceramics of solid solutions of the Ag7(Si1–xGex)S5I composition have been prepared. For the first time, the structural properties of the obtained solid solutions have been studied, the crystal structure has been modeled, and the mechanism of ion transport has been ascertained. Theoretical methods for predicting the physical properties within the framework of the density functional theory approach have been used to obtain the data set, which contributed to the optimization of the process of selecting the samples with the best properties for further research. For the first time, frequency-temperature studies of the electrical conductivity inherent to the studied samples have been carried out, and it has been ascertained that the temperature dependences of the ionic conductivity are linear and described by the Arrhenius law, which confirms the thermally activated nature of the conductivity. The measured dependences of electrical conductivity on frequency for the examined ceramic samples have typical behavior both for single crystals of solid solutions and for ceramic samples. For the first time, a study of the optical characteristics of single crystals of solid solutions of the Ag7(Si1–xGex)S5I composition has been carried out: the intrinsic absorption edge of single crystals of Ag7(Si1–xGex)S5I solid solutions has been determined within the temperature range 77–300 K. The value of the pseudogap has been calculated, and it has been ascertained that an increase in temperature leads to a monotonous decrease in its value. For the first time, the method of diffuse reflectance spectroscopy with further analysis by using the Kubelka–Munk function and the Tauc graphical method has been applied to estimate the pseudogap of these complex objects, namely, solid solutions based on compounds of the structure of the Ag7(Si1–xGex)S5I composition inherent to argyrodites. As a result of the research indicates disordering of the crystal structures of Ag7(Si1–xGex)S5I solid solutions as a result of cation substitution Si4+ ↔ Ge4+ within the anionic sublattice. The presence of a dimensional effect of microhardness during microindentation of single crystals and ceramic samples of Ag7Si1–xGexS5I solid solutions has been ascertained. It has been also ascertained that when the ion radius decreases at the Si4+ ↔ Ge4+ isovalent substitution, the microhardness of the crystals increases, which indicates an increase in the rigidity of the crystal structure in these materials.