Physical and optical properties of the quaternary sulfides SrCu2MS4 and EuCu2MS4 (M=Ge and Sn)

Four quaternary sulfides SrCu₂MS₄ and EuCu₂MS₄ (M=Ge and Sn) were prepared from a thoroughly ground mixture of EuS or SrS, Cu, or Sn, and S in stoichiometric proportions. Electrical conductivity measurements on pressed pellets showed that all the phases are semiconductors. The optical band gaps were assessed at 2.8 eV for SrCu₂GeS₄, 2.1 eV for SrCu₂SnS₄, 2.2 eV for EuCu₂SnS₄, and 1.6 eV for EuCu₂GeS₄. Both Sr-based compounds present a temperature-independent paramagnetism, of about +135×10⁻⁶ and +92×10⁻⁶ emu/mol, for SrCu₂SnS₄ and SrCu₂GeS₄, respectively. In the case of the europium compounds, they follow a Curie-Weiss dependence above 1.8 K (EuCu₂GeS₄) and above 4 K (for EuCu₂SnS₄), with values of the magnetic effective moment μ_eff and the Curie-Weiss temperature Θ, equal to 6.27 μ_B and −2.8 K for EuCu₂GeS₄, and 6.81 μ_B and +0.7 K, for EuCu₂SnS₄. The experimental magnetic moments confirm that the europium ion is in divalent state, similar to Sr in the related compounds.