SYNTHESIS AND STRUCTURE OF TERNARY CHALCOGENIDES OF RARE-EARTHS ALNX2 (A = ALKALINE METAL) (X = SULFUR, SELENIUM, TELLURIUM)

The sulfides RbLnS2 and CsLnS2 were synthesized by heating alkali carbonate and the respective rare earth oxides under an H2S atmosphere at 1000-degrees-C. The ternary sulfides form thin, mostly coloured plate-like crystals. The structures of the cesium compounds and RbCeS, were characterized by X-ray investigations on single crystals. All the compounds crystallize with the alpha-NaFeO2 Structure type; for CsLnS2 Compounds with Ln=Pr-Lu the beta-RbSCO2 Structure type was found as a second modification. This was proven by a single crystal X-ray investigation on CsPrS2. The samarium compounds NaSmSe2, KSmSe2, NaSmTe2, KSmTe2, and RbSmTe2 were prepared as examples for the corresponding selenides and tellurides. Syntheses were done by reacting the elements in a closed system. These chalcogenides also crystallize with the alpha-NaFeO2 structure type. NaSmSe2 shows an additional low temperature modification with a statistical distribution of the cations. For KSmTe2 the structural determination was carried out on a single crystal. The sequence of the lattice constants and the z-parameters of the sulfur atoms will be discussed in accordance to their dependence on the lanthanide contraction.