Syntheses, Structures, Physical Properties, and Theoretical Studies of CeM_xOS (M = Cu, Ag; x ≈ 0.8) and CeAgOS

Black single crystals of the two nonstoichiometric cerium coinage-metal oxysulfide compounds CeCu_xOS and CeAg_xOS (x ≈ 0.8) have been prepared by the reactions of Ce₂S₃ and CuO or Ag₂O at 1223 or 1173 K, respectively. A black powder sample of CeAgOS has been prepared by the stoichiometric reaction of Ce₂S₃, CeO₂, Ag₂S, and Ag at 1073 K. These isostructural materials crystallize in the ZrSiCuAs structure type with two formula units in the tetragonal space group P4/nmm. Refined crystal structure results and chemical analyses provide evidence that the previously known anomalously small unit-cell volume of LnCuOS for Ln = Ce (Ln = rare-earth metal) is the result of Cu vacancies and the concomitant presence of both Ce³⁺ and Ce⁴⁺. Both CeCu_0.8OS and CeAgOS are paramagnetic with μ_eff values of 2.13(6) and 2.10(1) μ_B, respectively. CeCu_0.8OS is a p-type semiconductor with a thermal activation energy E_a = 0.22 eV, σ_electrical = 9.8(1) 10^-3 S/cm at 298 K, and an optical band gap E_g < 0.73 eV. CeAgOS has conductivity σ_conductivity = 0.16(4) S/cm and an optical band gap E_g = 0.71 eV at 298 K. Theoretical calculations with an on-site Coulomb repulsion parameter indicate that the Ce 4f states are fully spin-polarized and are not localized in CeCuOS, CeCu_0.75OS, or CeAgOS. Calculated band gaps for CeCu_0.75OS and CeAgOS are 0.6 and 0.8 eV, respectively.