New Pressure-Induced Phase Transitions in Bismuthinite

The study of the effect of high pressure and high temperature on the crystal structure of Bi₂S₃ has revealed that Bi₂S₃ decays chemically at p = 6–8 GPa and T = 625–1050°C through the Bi₂S₃ → BiS₂ + Bi + S reaction with the formation of BiS₂-I (a = (15.74 ± 0.01) Å, b = (4.026 ± 0.004) Å, c = (11.34 ± 0.01) Å, $\beta = 127.51^\circ$, D_x = 6.36 g/cm³, V/Z = 71.31 ų), and a new metastable high-pressure phase BiS₂-II with mmm symmetry (a = (8.45 ± 0.01) Å, b = (10.26 ± 0.01) Å, c = (5.87 ± 0.007) Å, D_x = 7.12 g/cm³, V/Z = 63.695 Å³, ΔV/V₀ = 0.11) appears at p = 8–8.5 GPa and T ≥ 1100–1300°C. The BiS₂-II phase in mixture with the BiS₂-I phase is also formed in the Bi + S(1 : 2) composition at the same pressures and temperatures. Chalcogenides of the V₂VI₃ composition (V = As, Sb, Bi; VI = S, Se) demonstrate a pronounced tendency to thermal instability with the formation of chalcogenides with “incorrect” VVI₂ stoichiometry such as AsS₂, AsSe₂, BiS₂-I, BiS₂-II, and BiSe₂, but the layered structures of these metastable high-pressure phases are different. It can be assumed that the pressure-induced V₂VI₃ → VVI₂ + VVI and V₂VI₃ → VVI₂ + V + VI decay reactions into compounds with incorrect stoichiometry are a general property of these chalcogenides.