The CsLnMSe₃ semiconductors (Ln = rare-earth element, Y; M = Zn, Cd, Hg)

CsLnCdSe₃ (Ln = Ce, Pr, Sm, Gd, Tb, Dy, Y) and CsLnHgSe₃ (Ln = La, Ce, Pr, Nd, Sm, Gd, Y) have been synthesized at 1123 K. These isostructural materials crystallize in the layered KZrCuS₃ structure type in the orthorhombic space group Cmcm and are group X extensions of the previously characterized Zn compounds. The structure is composed of two-dimensional _∞²[LnMSe₃] layers that stack perpendicular to [010] and are separated by layers of face- and edge-sharing CsSe₈ bicapped trigonal prisms. Because there are no Se-Se bonds in the structure of CsLnMSe₃ (M = Zn, Cd, Hg), the formal oxidation states of Cs/Ln/M/Se are 1+/3+/2+/2-. CsSmHgSe₃ does not adhere to the Curie-Weiss law, whereas CsCeHgSe₃ and CsGdHgSe₃ are Curie-Weiss paramagnets with μ_eff values of 2.77 and 7.90 μ_B, corresponding well with the theoretical values of 2.54 and 7.94 μ_B for Ce³⁺ and Gd³⁺, respectively. Single-crystal optical absorption measurements were performed with polarized light perpendicular to the (010) and (001) crystal faces of these materials. The band gaps of the (010) crystal faces range from 1.94 eV (CsCeHgSe₃) to 2.58 eV (CsYCdSe₃) whereas those of the (001) crystal faces span the range 2.37 eV (CsSmHgSe₃) to 2.54 eV (CsYCdSe₃ and CsYHgSe₃). The largest band gap variation between crystal faces is 0.06 eV for CsYCdSe₃. Theoretical calculations for CsYMSe₃ indicate that these materials are direct band gap semiconductors whose colors and optical band gaps are dependent upon the orbitals of Y, M, and Se.