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

The sulfides RbLnS2 and CsLnS2 were synthesized by heating alkali carb onate 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 comp ounds crystallize with the alpha-NaFeO2 Structure type; for CsLnS2 Com pounds with Ln=Pr-Lu the beta-RbSCO2 Structure type was found as a sec ond modification. This was proven by a single crystal X-ray investigat ion on CsPrS2. The samarium compounds NaSmSe2, KSmSe2, NaSmTe2, KSmTe2 , and RbSmTe2 were prepared as examples for the corresponding selenide s and tellurides. Syntheses were done by reacting the elements in a cl osed system. These chalcogenides also crystallize with the alpha-NaFeO 2 structure type. NaSmSe2 shows an additional low temperature modifica tion with a statistical distribution of the cations. For KSmTe2 the st ructural determination was carried out on a single crystal. The sequen ce of the lattice constants and the z-parameters of the sulfur atoms w ill be discussed in accordance to their dependence on the lanthanide c ontraction.