NEW PICTURE OF PHASE EQUILIBRIA IN THE Cu-As-Se SYSTEM AND THERMODYNAMIC PROPERTIES OF TERNARY COMPOUNDS

A new complete picture of phase equilibria in the Cu-As-Se system was obtained by experimentally studying carefully crystallized alloys by prolonged thermal annealing using differential thermal analysis and powder X-ray diffraction methods, as well as considering information found in the literature. The liquidus surface projection, isothermal section at 300 K and some vertical sections of the phase diagram are presented and discussed. The types and coordinates of nonvariant and monovariant phase equilibria are determined. It is established that the liquidus surface consists of 15 regions corresponding to the primary crystallization of three initial components, seven binary and five ternary compounds. The presented phase diagram reflects the compounds Cu3AsSе4, Cu3AsSе3, Cu7As6Sе13, CuAsSе2 and CuAsSе, which can be considered as synthetic analogues of natural copper-arsenic minerals. The ternary compounds Cu6As4Se9 and Cu4As2Se5, indicated in the literature, have not been confirmed by us. The thermodynamic properties of ternary copper-arsenic selenides were studied using the EMF method with the solid electrolyte Cu4RbCl3I2. From the EMF measurement data in the corresponding three-phase regions, linear equations of the temperature dependence of the EMF were obtained, from which partial thermodynamic functions of copper in the alloys were calculated. Based on these data and the solid-phase equilibrium diagram of the Cu-As-Se system, using the corresponding thermodynamic functions of copper, As2Se3, AsSe and Cu3AsSe4, the standard Gibbs free energy of formation and the enthalpy of formation, as well as the standard entropy of the ternary compounds Cu3AsSе3, Cu7As6Sе13, CuAsSе2 and CuAsSе, were calculated.