Experimentally determined thermodynamic properties of schapbachite (α-AgBiS₂) below T = 700 K

Thermodynamic properties of schapbachite (α-AgBiS₂) in the phase assemblage α-AgBiS₂-AgBi₃S₅-Bi have been studied by an EMF-technique. The EMF-measurements were made on the galvanic cell Pt(-)|Ag|AgI|AgBiS₂ + AgBi₃S₅ + Bi|C|Pt(+), over the temperature range from (429 to 699) K. According to the EMF vs. temperature relations obtained, the enthalpy of the phase transformation from β-AgBi_1+xS₂ to α-AgBi _1+xS₂, at T = (465.55 ± 5) K, was calculated to be (7.3 ± 2.1) kJ · mol^-1. New experimentally determined thermodynamic properties of the bismuth-saturated schapbachite (α-AgBi_1+xS₂), for each temperature region of the stable phases Bi(s) and Bi(l), were generated and analysed in detail. Based on the experimental results, Gibbs free energies of sulfidation reactions including Ag, Bi(l), S₂(g), Ag₂S and Bi₂S₃ to produce the bismuth-saturated schapbachite (α-AgBi_1+xS₂) have been evaluated. It has been observed that within the temperature range from (474 to 680) K, schapbachite saturated with bismuth (α-AgBi_1+xS ₂) is thermodynamically more stable than the stoichiometric schapbachite (α-AgBiS₂).