A thermodynamic study of digenite solid solution (cu2-δs) at 600 to 1000°c and a statistical thermodynamic critique on general nonstoichiometry

Abstract

A very accurate experimental method to determine the composition of metal-saturated sulfides has been developed and applied to Cu2-δS. The technique consists of using a high-sensitivity thermobalance which operates in a controlled atmosphere of H2-H2S gas. By this method it was shown that Cu-saturated CuxS is stoichiometric(x= 2.0000 ± 0.0002) between 700 and 1000°C. The free energy of formation for Cu2S was found to be: 2Cu(s) + 1/2 S2(g) = Cu2.0000S(s), ΔG0 = -30,610 + 6.80T (cal/mole). The sulfur partial pressure was determined over nonstoichiometric Cu2-δS for sulfur contents up to 21 pct. From the result, thermodynamic functions such as activity of copper, heats and entropies of solution were calculated as a function of nonstoichiometric composition. It was thermo-dynamically demonstrated that previous models dealing with nonstoichiometric oxides (or sulfides) based on the law of mass action,i.e., δ = const ie 67 01 (or ie 67 02), are inconsis-tent as they fail to satisfy the Gibbs-Duhem relation and also fail to account for the dis-sociation pressure over the stoichiometric composition. To resolve this dilemma, a sta-tistical thermodynamic method of constructing the grand partition function was introduced. Stoichiometry and nonstoichiometry of the Cu2-δS were then explained by postulating an ionic crystal consisting of Cu+, Cu2+, Cu0 and neutral vacancies in the copper sublattice and S2− and neutral vacancies in the sulfur sublattice.