Crystal structure and elementary properties of PbS₂ with a pressure–stabilized S-S dimer

PbS₂ and Pb_0.9Ag_0.1S₂ were prepared through the reaction of PbS with excess sulfur (and silver) at 4 GPa and 600 °C. PbS₂ crystallizes in the CuAl₂ structure type, (I4/mcm (#140), a = 6.1106(5) Å c = 7.4949(6) Å Z = 4), determined by single crystal X-ray diffraction. Its structure consists of layers of [S₂]^2- dimers and Pb²⁺ in square antiprismatic coordination – a rare structure type for metal dichalcogenides. Electronic structure calculations suggest that the material should be an indirect band gap semiconductor. Consistent with the calculations, the temperature-dependent resistivity of undoped PbS₂ is that of a degenerate semiconductor, while the Ag-doped material shows metallic temperature dependent resistivity down to 2 K. The materials are both diamagnetic. For the Ag-doped material, the Seebeck coefficient is small and shows nearly linear temperature-dependent behavior from 50 to 250 K.