Syntheses, Structure, Magnetism, and Optical Properties of the Ordered Interlanthanide Copper Chalcogenides Ln₂YbCuQ₅ (Ln = La, Ce, Pr, Nd, Sm; Q = S, Se): Evidence for Unusual Magnetic Ordering in Sm₂YbCuS₅

Ln 2 YbCuQ 5 (Ln = La, Ce, Pr, Nd, Sm; Q = S, Se) have been prepared by direct reaction of the elements in Sb 2 Q 3 (Q = S, Se) fluxes at 900 °C. All compounds have been characterized by single-crystal X-ray diffraction methods and are isotypic. The structure of Ln 2 YbCuQ 5 consists of one-dimensional ∞ 1 [YbCuQ 5 ] 6− ribbons extending along the b -axis that are connected by larger Ln 3+ ions. Each ribbon is constructed from two single chains of [YbQ 6 ] octahedra with one double chain of [CuQ 5 ] trigonal bipyramids in the middle. All three chains connect with each other via edge-sharing. There are two crystallographically unique Ln atoms, one octahedral Yb site, and two disordered Cu positions inside of distorted Q 5 trigonal bipyramids. Both Ln atoms are surrounded by eight Q atoms in bicapped trigonal prisms. The magnetic properties of Ln 2 YbCuQ 5 have been characterized using magnetic susceptibility and heat capacity measurements, while their optical properties have been explored using UV−vis−NIR diffuse reflectance spectroscopy. Ce 2 YbCuSe 5, La 2 YbCuS 5, Ce 2 YbCuS 5, and Pr 2 YbCuS 5 are Curie−Weiss paramagnets. La 2 YbCuSe 5 and Nd 2 YbCuS 5 show evidence for short-range antiferromagnetic ordering at low temperatures. Sm 2 YbCuS 5 shows magnetic ordering at 5.9 K, followed by negative magnetization at low external fields. The band gaps of La 2 YbCuSe 5, Ce 2 YbCuSe 5, La 2 YbCuS 5, Ce 2 YbCuS 5, Pr 2 YbCuS 5, Nd 2 YbCuS 5, and Sm 2 YbCuS 5 are 1.15, 1.05, 1.45, 1.37, 1.25, 1.35, and 1.28 eV, respectively.