Calculation of thermodynamic functions of saturated solid solution of AgIn 2 Te 3 I compound in the Ag – In – Te – I system

. Triangulation of Ag – In – Te – I system in the vicinity of AgIn 2 Te 3 I compound was investigated by X-ray diffraction and differential thermal analysis methods. The spatial position of the phase region AgIn 2 Te 3 I – InTe – Ag 2 Te – AgI regarding the figurative point of silver was used in order to write the equation of virtual potential-forming reaction. Potential-forming reaction was performed in electrochemical cell (ECC) of the type ( – ) C | Ag | Ag 3 GeS 3 I(Br) glass | D | C (+) where C are inert (graphite) electrodes; Ag and D are the electrodes of the ECC; D represents the alloy of four-phase region; Ag 3 GeS 3 I glass is a membrane with purely ionic Ag + conductivity). Linear dependence of the EMF of cell on temperature in the range of 440 – 480 К was used to calculate the standard thermodynamic functions of saturated solid solution of AgIn 2 Te 3 I compound in Ag – In – Te – I system.


Introduction
In recent years, there has been an increasing interest in solid ionic conductors in view of the need to develop novel devices such as micro-and nano supercapacitors, ion selective membranes, batteries, fuel cells, sensors, electrochromic displays, etc [1,2]. The AgIn 2 X 3 Y compounds (X -S, Se, Te; Y -Cl, Br, I) belong to the group of silver-containing chalcogenide crystalline Ag 3 SI(Br) and glassy Ag 3 GeS 3 I(Br) superionic phases in which chalogenide anions, along with silver cations, possess the properties of non-stationary quasi-liquids [3]. This paper presents the results of the experimental determination of the boundaries of phase regions including the AgIn 2 Te 3 I compound, and calculation of thermodynamic functions of saturated solid solution of the four-element compound.

Experimental
The InTe, In 2 Te 3 , Ag 2 Te compounds were prepared by cooling of the melts of the mix of the elements of semiconductor purity. The Ag 3 GeS 3 I glass was obtained by quenching the melt of elements and AgI from 1200 K into ice water. Crystals of indium and silver tellurides (particle size ≤5 μm) together with powdered AgI were used for making the positive electrodes of electrochemical cell (ECC) and for alloys for differential thermal analysis (DTA) and X-ray diffraction (XRD). Well-mixed blends of calculated quantities of the compounds were placed into quartz ampoules and evacuated to the residual pressure ~ 1 Pa. Solid-phase synthesis was performed at 820 K for 20 days. The phase composition of the alloys was determined by DTA and XRD. XRD patterns were collected on a STOE STADI P diffractometer [4] equipped with a linear positionsensitive detector PSD, in a modified Guinier geometry (transmission mode, CuKα 1 radiation, a bent Ge (111) monochromator, 2θ/ω scan mode). XRD arrays were processed using STOE WinX POW (version 2.21) and PowderCell (version 2.3) software suits.
Potential-forming process was performed in electrochemical cell of the type (-) C | Ag | Ag 3 GeS 3 I glass | D | C (+) where C are the inert graphite electrodes; Ag and D are the electrodes of ECC; D represents the alloy of four-phase region; and Ag 3 GeS 3 I glass is a membrane with purely ionic Ag + conductivity.
Powdered cell components were pressed (р ~ 10 8 Pa) into through holes with the diameter of 2 mm arranged in the PTFE matrix up to the density ρ = (0.93 ± 0.02)ρ 0 , where ρ 0 is the experimentally determined density of cast alloys. To eliminate the defects of plastic deformation under extrusion of alloys, we performed five-fold thermal cycling of ECC in the range of 400-480 K with heating and cooling rates of 2 K min -1 . The ECC was heated in a resistance furnace filled with a mixture of H 2 and Ar in a molar ratio of 1 : 9, p = 1.2×10 5 Pa. The flow of gas at the rate of 2×10 -3 m 3 h -1 had a direction from the positive to the negative electrode of the ECC. The temperature was maintained with an accuracy of ±0.5 K.
The EMF values of the cells were measured using the voltmeter of a U7-9 electrometric amplifier with an input resistance of >10 12 Ω.

Results and discussion
According to Fig. 1, virtual chemical reaction in the ECC involving four-element phase is described by the equation 2Ag + AgIn 2 Te 3 I = AgI + 2InTe + Ag 2 Te. (1)