Chekaylo M. Modification of Ag8SiSe6, Ag8SiSe6 and Ag8SnSe6 Argyrodites Structure and Properties, for Electronic Engineering

The processes of synthesis of the three argyrodite compounds (Ag8SiSe6, Ag8SiSe6 and Ag8SnSe6) are investigated by means of DTA method. Phase transitions, chemical reactions of step-by-step synthesis and their temperature ranges are identified. Physical-technological foundations of the compound synthesis are developed. The foundations are based on the establishment of the fact that the synthesis from elementary components consists of two stages: at the first step binary compounds of Ag2Se and XSe2 (X = Si, Ge, Sn) and later ternary compounds of argyrodite are synthesized. In the complex way by means of dilatometry, scanning calorimetry, thermal X-ray structural analysis and Mossbauer spectroscopy methods, the properties of the grown monocrystals of Ag8XSe6 (X = Si, Ge, Sn) argyrodites have been investigated, and it is shown that essential polymorphy (three PT in Ag8SiSe6, Ag8SiSe6 and one PT in Ag8SnSe6) and mixed (electronic-ionic) conduction are characteristic of them and they are solid electrolytes. Solid electrochemical cells (SEC) which are manufactured on their basic show resistance-time dependence in potentiostatic mode, and in the mode of volt-ampere characteristic they show low resistance and high resistance states, between which switch effect takes place. Fundamental physical-chemical parameters of argyrodites i. e.: enthalpies of compound formation, the enthalpies of chemical reactions of their synthesis and those of fusion are determined. For the first time, the enthalpies and entropies of the observed PT, which are important in scientific information aspect, are determined and application of argyrodites as materials for electronic engineering, are found. For the first time, in the course of calorimetric investigation of Ag8SiSe6 argyrodites, at the temperature T = 353 K a third PT (of first kind) was discovered. For the first time, with the use of X-ray structure analysis, the crystalline structure of low-temperature -Ag8SnSe6 (space group Pmn21 (No 31)) and high-temperature -Ag8SnSe6 (space group (No 216)) modifications of argyrodite are identified. In -Ag8SnSe6 and -Ag8SnSe6 modifications of argyrodite, incomplete (partial) occupation of Ag-atom positions is discovered; that serves as a structural basic of their ionic conduction. С/Ag8ХSe6/C (Х = Sn, Ge) SEC is created, curves I(t) are investigated in potentiostatic mode, an algorithm of description of the dependence is suggested. On the basis of these, equivalent scheme of SEC is identified.