New routes to alkali-metal–rare-earth-metal sulfides

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Abstract
Ternary alkali-metal–rare-earth-metal sulfides, MRES2(M = Li, Na, K; RE = Y, La–Yb) were prepared by heating RECL3and M2S under an H2S–N2 atmosphere at 750–800 °C for 10 min. The ternary sulfides form thin, mostly coloured plates. The structures were characterised by X-ray powder diffraction, which revealed high-temperature cubic modifications for LiHoS2 and LiErS2. The disordered cubic form (NaCl type) was observed for LiLnS2(Ln = Pr–Er) and NaLnS2(Ln = La–Nd), while the ordered rhombohedral form (α-NaFeO2 type) was observed for LiYbS2, NaLnS2(Sm–Yb) and KLnS2(La–Yb). Alkali-metal–rare-earth-metal sulfides may also be synthesized from the rare-earth-metal chloride and alkali-metal halide under comparable conditions. Similarly, thermolysis of the rare-earth-metal sesquisulfide (γ-RE2S3), oxysulfide (RE2O2S) or oxychloride (REOCL) with M2S or MCI in an atmosphere of H2S-N2 gives MRES2. The materials were characterised by powder X-ray diffraction, scanning electron microscopy (SEM), energy dispersive analysis by X-rays (EDXA), Raman, infrared, magnetic moments and microanalysis.