Redetermination of NaGdS2, NaLuS2and NaYS2

The title structures NaGdS2(sodium gadolinium sulfide), NaLuS2(sodium lutetium sulfide) and NaYS2(sodium yttrium sulfide) were redetermined in order to improve the structural information available for the family of group 1 and thallium rare earth sulfides, which are isostructural with the rhombohedral α-NaFeO2structure type. In particular, the present investigation has been directed at the rhombohedral sodium rare earth sulfides. The observed dependence of the fractional coordinatez(S2−) on the identity of the rare earth element in the newly determined structures is in agreement with the known structures of the potassium and rubidium analogues. Crystals of NaGdS2and NaLuS2display obverse–reverse twinning.