Cs₂Ln₃CuS₈ (Ln = La–Nd, Sm–Tb): Synthesis, Crystal Structure, and Magnetic and Optical Properties

This work reports the preparation of new quaternary sulfides Cs2Ln3CuS8 (Ln = La–Nd, Sm–Tb), their original crystal and electronic structures, and their magnetic properties. The sulfides were prepared using a reactive flux method from mixtures of Ln2S3 (EuS), Cs2S6, Cu2S, and S. They crystallize in a new type of structure (C2/m space group) and exhibit a layer-like crystal structure, which is a hybrid of those of the ACe2CuS6 series (A = Cs, K) and that of K2CeCu2S4. The values of the optical band gap calculated by the Kubelka–Munk equation are in the range of 1.2–2.62 eV depending on the nature of the Ln ion. The Cs2Gd3CuS8 compound displays relatively great magnetic refrigerating properties at cryogenic temperature with the mass entropy change (−ΔSM) reaching 19.5 J kg–1 K–1 at 3.5 K for ΔH = 5 T.