TERNARY RUBIDIUM RARE-EARTH SULFIDES - SYNTHESIS AND STRUCTURE

The sulphides RbLnS(2), where Ln = rare earth metal, could be synthesi zed by heating a mixture of rubidium carbonate and the selected rare e arth sesquioxide in a stream of H2S. The ternary sulphides form thin m ostly coloured plate-like crystals. All compounds crystallize in the a lpha-NaFeO2 structure type. Relations between the cell parameters as w ell as the z parameters of the sulphur atoms and the atomic number of the rare earth metals are discussed. The quotients U-11:U-33 of the th ermal parameters seem to be nearly constant for the atoms A, B and X i n compounds ABX(2) crystallizing in the alpha-NaFeO2 structure type. T he Madelung Part of Lattice Energy [MAPLE] is discussed in detail. All ternary sulphides RbLnS(2) and CsLnS(2) are MAPLE-exothermic.