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) Angstrom, b = 7.7692(9) Angstrom, c = 7.4665(9) An gstrom, and beta = 95.045(2)degrees (T = 153 K) with four formula units in space group C-2h(5)-P2(l)/c of the monoclinic system. Its structure is clos ely related to that of LaCuS2. SmCuSTe crystallizes as a new structure type in a cell of dimensions 7.5195(8) Angstrom, b = 12.6370(14) Angstrom, and c 7.5287(8) Angstrom (T = 153 K) with eight formula units in space group D- 2h(15)-Pbca of the orthorhombic system. In LaCuSTe. the La atom is coordina ted by a bicapped trigonal prism of four S atoms and Four Te atoms; in SmCu STe, the Sm atom is coordinated by a monocapped trigonal prism of four S at oms 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 f rom (CuSTe)(n) sheets that are separated by the rare-earth-metal atoms. In LaCuSTe, a sheet comprises edge-sharing dimers of CuSTe3 tetrahedra (Cu2S2T e4) 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 neighb oring tetrahedra to Form a hexagonal net. The band structures of these two compounds indicate that they should be semiconductors. The calculations als o show that the substitution of Te atoms for S atoms lowers the energy gap compared with those of unsubstituted systems.