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

• A galvanic cell incorporating AgI as the solid electrolyte was measured accurately.
• Thermodynamics of the phase assemblage AgBiS2–AgBi3S5–Bi have been investigated.
• ΔfG∘ values for bismuth-saturated schapbachite (α-AgBi1+xS2) have been determined.
• New experimental thermodynamic data for Bi-saturated schapbachite were generated.
• ΔrG∘ values for reactions involving S2(g) to produce α-AgBi1+xS2 have been evaluated.

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-AgBi1+xS2 to α-AgBi1+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-AgBi1+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-AgBi1+xS2) have been evaluated. It has been observed that within the temperature range from (474 to 680) K, schapbachite saturated with bismuth (α-AgBi1+xS2) is thermodynamically more stable than the stoichiometric schapbachite (α-AgBiS2).