Phase Relations in the Cu-As-S System and Thermodynamic Properties of Copper-Arsenic Sulfides

Here, a complete phase equilibria picture in the Cu-As-S system was obtained by experimental study of carefully crystallized via long-term thermal annealing alloys by means of methods of differential thermal analysis and powder x-ray diffraction, as well as using the available literature data. The projection of the liquidus surface, the isothermal section at 300 K, and some vertical sections of the phase diagram are presented and discussed. The fields of primary crystallization of phases, types, and coordinates of invariant and monovariant phase equilibria are determined. The presented phase diagram reflects four ternary compounds Cu₃AsS₄, Cu₁₂As₄S₁₃, Cu₆As₄S₉, and CuAsS, which are synthetic analogues of natural copper-arsenic sulfide minerals. Particular attention is paid to the Cu₂S-As₂S₃ section. It is shown that this section, in contrast to the literature data, is not quasi-binary. The thermodynamic data for copper-arsenic sulfides, previously obtained by the authors by the electromotive force method with Cu₄RbCl₃I₂ solid electrolyte, have also been revised. Experimental data on the partial thermodynamic functions of copper in some phase regions of the Cu-As-S system were processed taking into account the constructed new version of the solid-phase equilibria diagram and updated data on the standard thermodynamic functions of formation and standard entropies of the ternary compounds Cu₃AsS₄, Cu₁₂As₄S₁₃, Cu₆As₄S₉, and CuAsS were obtained.