Syntheses, Structure, Magnetism, and Optical Properties of the Ordered Mixed-Lanthanide Sulfides γ-LnLn‘S₃ (Ln = La, Ce; Ln‘ = Er, Tm, Yb)

γ-LnLn‘S₃ (Ln = La, Ce; Ln‘ = Er, Tm, Yb) have been prepared as dark red to black single crystals by the reaction of the respective lanthanides with sulfur in a Sb₂S₃ flux at 1000 °C. This isotypic series of compounds adopts a layered structure that consists of the smaller lanthanides (Er, Tm, and Yb) bound by sulfide in six- and seven-coordinate environments that are connected together by the larger lanthanides (La and Ce) in eight- and nine-coordinate environments. The layers can be broken down into three distinct one-dimensional substructures containing three crystallographically unique Ln‘ centers. The first of these is constructed from one-dimensional chains of edge-sharing [Ln‘S₇] monocapped trigonal prisms that are joined to equivalent chains via edge sharing to yield ribbons. There are parallel chains of [Ln‘S₆] distorted octahedra that are linked to the first ribbons through corner sharing. These latter units also share corners with a one-dimensional ribbon composed of parallel chains of [Ln‘S₆] polyhedra that edge-share both in the direction of chain propagation and with adjacent identical chains. Magnetic susceptibility measurements show Curie−Weiss behavior from 2 to 300 K with antiferromagnetic coupling and no evidence for magnetic ordering. The θ_p values range from −0.4 to −37.5 K, and spin-frustration may be indicated for the Yb-containing compounds. All compounds show magnetic moments substantially reduced from those calculated for the free ions. The optical band gaps for γ-LaLn‘S₃ (Ln‘ = Er, Tm, Yb) are approximately 1.6 eV, whereas γ-CeLn‘S₃ (Ln‘ = Er, Tm, Yb) are approximately 1.3 eV.