Preparation and structure of sodium rare-earth sulfides, NaLnS2 (Ln; rare earth elements)

Ternary sodium rare-earth sulfides, NaLnS2 (Ln; rare earth elements), have been prepared and characterized. With an x-ray diffraction analysis of their crystal structure, it has been found that the materials containing La, Ce, and Pr crystallize in a cubic structure, including a Th3P4-type compound as a second phase, while the other NaLnS2 sulfides - except for NaNdS2 and NaSmS2 - crystallize in a rhombohedral NaHF2-type structure, which can be described with a hexagonal unit cell. In NaNdS2 and NaSmS2 these two phases, the cubic and the rhombohedral, coexist. The following relationships can be obtained from a comparision of the lattice constants of the cubic and hexagonal cells in NaNdS2; ah=ac√22, ch=ac2 √3. Based on these relationships, the structural type of the cubic phase can be regarded as analogous to that of NaCl.