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 (infinity)[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 (mu)eff values of 2.77 and 7.90 (mu)B, corresponding well with the theoretical values of 2.54 and 7.94 (mu)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.