Syntheses, Structures, and Theoretical Study of LaCuSTe and SmCuSTe

Two mixed-chalcogen compounds LaCuSTe and SmCuSTe have been synthesized by the reactions of the elements at 1123 K. LaCuSTe crystallizes in a cell of dimensions a = 6.4955(8) Å, b = 7.7692(9) Å, c = 7.4665(9) Å, and β = 95.045(2)° (T = 153 K) with four formula units in space group C2h5-P21/c of the monoclinic system. Its structure is closely related to that of LaCuS2. SmCuSTe crystallizes as a new structure type in a cell of dimensions a = 7.5195(8) Å, b = 12.6370(14) Å, and c = 7.5287(8) Å (T = 153 K) with eight formula units in space group D2h15-Pbca of the orthorhombic system. In LaCuSTe, the La atom is coordinated by a bicapped trigonal prism of four S atoms and four Te atoms; in SmCuSTe, the Sm atom is coordinated by a monocapped trigonal prism of four S atoms and three Te atoms. In both structures, the Cu atom is coordinated by a tetrahedron of one S atom and three Te atoms, These structures are built from (CuSTe)n sheets that are separated by the rare-earth-metal atoms. In LaCuSTe, a sheet comprises edge-sharing dimers of CuSTe3 tetrahedra (Cu2S2Te4) that share each of their Te atoms with four neighboring dimers to form a mixed octagonal/quadrilateral net. In SmCuSTe, a sheet comprises a single tetrahedral CuSTe3 unit that shares each of its Te atoms with three neighboring tetrahedra to form a hexagonal net. The band structures of these two compounds indicate that they should be semiconductors. The calculations also show that the substitution of Te atoms for S atoms lowers the energy gap compared with those of unsubstituted systems.