Unravelling the rare-earth (RE) element-induced magnetic and optical properties in the structures of quaternary selenides SrRECuSe3

We report novel selenides SrRECuSe3 (RE = Nd, Sm, Tb-Yb), which crystal structures complete the series of the whole row of SrRECuSe3 (RE = Sc, Y, La-Nd, Sm, Gd-Lu). Polycrystalline samples were obtained by reductive selenidation with stepwise heating from 870 to 1070 K. Selenides SrRECuSe3 (RE = Y, Gd, Lu) were also obtained using the same synthetic approach. The resulting compounds crystallize in two space groups, namely Pnma (RE = Y, Nd, Sm, Gd-Dy) and Cmcm (RE = Sc, Ho-Lu). Both types of crystal structures consist of the RESe6 octahedra and CuSe4 tetrahedra but differ in the coordination polyhedron of the Sr2+ cation. In SrRECuSe3 (RE = Y, Nd, Sm, Gd-Dy), the Sr2+ cation is characterized by a monocapped trigonal prism, while in SrRECuSe3 (RE = Sc, Ho-Lu) the coordination environment of Sr2+ is described by highly symmetrical trigonal prismatic coordination. Both structural types are formed by layers separated either by chains formed by monocapped trigonal prisms or by alternating chains formed by trigonal prisms. The band gap value of SrRECuSe3 varies from 1.45 eV to 2.21 eV. Selenides SrRECuSe3 (RE = Sm, Gd, Tb, Ho, Er, Tm, Yb) at temperatures above 4 K behaves like paramagnets with antiferromagnetic interaction of the RE3+ moments. The experimental Curie constants and effective magnetic moments are in good agreement with the calculated ones for RE3+ ions excluding Sm3+.