Syntheses, structures, and physical properties of the new quaternary rare-earth chalcogenides RbNd₂CuS₄, RbSm₂CuS₄, CsLa₂CuSe₄, CsSm₂CuSe₄, RbEr₂Cu₃S₅, CsGd₂Ag₃Se₅, CsTb₂Ag₃Se₅

Eight quaternary rare-earth chalcogenides, RbNd₂CuS₄, RbSm₂CuS₄, CsLa₂CuSe₄, CsSm₂CuSe₄, RbEr₂Cu₃S₅, CsGd₂Ag₃Se₅, CsTb₂Ag₃Se₅, and Rb₂Gd₄Cu₄S₉, have been synthesized at 973 K with the use of a reactive flux of A₂Q₃ (A = Rb, Cs; Q = S, Se). All structural data were obtained at 153 K. The isostructural compounds ALn₂CuQ₄ (Ln =rare earth) crystallize with four formula units in the KGd₂CuS₄ structure type in space group Cmcm of the orthorhombic system; the isostructural compounds ALn₂M₃Q₅ (M = Cu, Ag) crystallize with four formula units in the RbSm₂Ag₃Se₅ structure type in space group Cmcm of the orthorhombic system; and Rb₂Gd₄Cu₄S₉ crystallizes with two formula units in space group C2/m of the monoclinic system. The cell dimensions of ALn₂CuQ₄ (a, b, c (Å)) are as follows: RbNd₂CuS₄, 4.0762(3), 13.954(1), 13.964(1); RbSm₂CuS₄, 4.0391(3), 13.815(1), 13.860(1); CsLa₂CuSe₄, 4.3129(6), 14.959(2), 14.798(2); CsSm₂CuSe₄, 4.2066(3), 14.6101(9), 14.5164(9). The corresponding R₁ indices for the refined structures are 0.0264, 0.0196, 0.0258, and 0.0224. The cell dimensions of ALn₂M₃Q₅ are as follows: RbEr₂Cu₃S₅, 3.9283(3), 13.897(1), 16.348(1); CsGd₂Ag₃Se₅, 4.2943(4), 15.424(1), 17.501(2); CsTb₂Ag₃Se₅, 4.2779(4), 15.429(2), 17.426(2). The R₁ indices are 0.0257, 0.0255, and 0.0241. The cell dimensions of of Rb₂Gd₄Cu₄S₉ (a, b, c (Å), β (°)) is 13.897(1), 3.9883(3), 16.054(1), 109.273(1), and the R₁ index is 0.0199. All eight compounds have closely related three-dimensional tunnel structures. All alkali metal atoms in the tunnels are coordinated to eight Q atoms. Their anionic frameworks are built from LnQ₆ octahedra and MQ₄ tetrahedra. ALn₂CuQ₄ contains _∞¹[CuQ₃] chains of vertex-sharing tetrahedra; Rb₂Gd₄Cu₄S₉ contains _∞¹[Cu₄S₈] chains of tetrahedra; and ALn₂M₃Q₅ contains _∞²[M₃Q₅] layers of tetrahedra. Rb₂Gd₄Cu₄S₉ and RbEr₂Cu₃S₅, which are paramagnetic, obey the Curie-Weiss law, and have effective magnetic moments of 7.9(2) μ_B for Gd³⁺ and 9.43(5) μ_B for Er³⁺. A band gap of 1.94 eV for Rb₂Gd₄Cu₄S₉ was deduced from its diffuse reflectance spectrum, and the typical 4f-4f optical transitions for Er³⁺ were found in RbEr₂Cu₃S₅ at about 1.6, 1.9, and 2.5 eV.