New layered materials: Syntheses, structures, and optical and magnetic properties of CsGdZnSe₃, CsZrCuSe₃, CsUCuSe₃, and BaGdCuSe₃

Four new quaternary selenides CsGdZnSe₃, CsZrCuSe₃, CsUCuSe₃, and BaGdCuSe₃ have been synthesized with the use of traditional high-temperature solid-state experimental methods. These compounds are isostructural with KZrCuS₃, crystallizing with four formula units in the orthorhombic space group Cmcm. Cell constants (Å) at 153 K are CsGdZnSe₃ 4.1684(7), 15.765(3), 11.0089(18), CsZrCuSe₃ 3.903(2), 15.841(10), 10.215(6), CsUCuSe₃ 4.1443(7), 15.786(3), 10.7188(18); and BaGdCuSe₃ 4.1839(6), 13.8935(19), 10.6692(15). The structure of these ALnMSe₃ compounds (A = Cs, Ba, Ln = Zr, Gd, U; M = Cu, Zn) is composed of _∞ ²[LnMSe₃ ^n-] (n = 1, 2) layers separated by A atoms. The Ln atom is octahedrally coordinated to six Se atoms, the M atom is tetrahedrally coordinated to four Se atoms, and the A atom is coordinated to a bicapped trigonal prism of eight Se atoms. Because there are no Se-Se bonds in the structure, the oxidation state of A is 1+ (Cs) or 2+ (Ba), that of Ln is 3+ (Gd) or 4+ (Zr, U), and that of M is 1+ (Cu) or 2+ (Zn). CsGdZnSe₃ and BaGdCuSe₃, which are paramagnetic, obey the Curie-Weiss law and have effective magnetic moments of 7.87(6) and 7.85(5) μ_B for Gd³⁺, in good agreement with the theoretical value of 7.94 μ_B. Optical transitions at 1.88 and 2.92 eV for CsGdZnSe₃ and 1.96 eV for BaGdCuSe₃ were deduced from diffuse reflectance spectra.