Comparative study of some rare earth sulfides: doped γ-[A]M2S3 (M = La, Ce and Nd, A = Na, K and Ca) and undoped γ-M2S3 (M = La, Ce and Nd)

The crystal structure determination of γ-M2S3 compounds (M = La, Ce, and Nd) has been carried out for the first time from single crystals obtained through high-temperature melting under sulfur pressure. The three phase structures do not depart from the cubic Th3P4 structural type, with a statistical filling of the dodecahedral sites by the cations. The γ-Na0.5Ce2.5S4-doped phase structure has also been determined from a powder neutron diffraction study. Na+ was found to be located at the dodecahedral site, in agreement with the composition limit of Na/Ce = 0.20 as determined by cell parameter variation versus composition. A powder X-ray diffraction study of the potassium- and calcium-doped derivatives (γ-K0.46C2.54S4 and γ-Ca0.89Ce2.07S4) confirmed the results obtained for the sodium-doped phase. In no case, at least in the phases studied, does the alkali or alkaline earth metal occupy the inter-dodecahedral tetrahedral sites. The electronic band structures of Ce2S3 and of Ce3−xS4 (0 < x <13) indicate an insulating behavior for the former compound and a metallic behavior for the latter, assuming in this case a rigid band model. In Ce3S4, the electronic conductivity takes place along the CeCe bonds. No SS bonding was found in the two binaries. It seems possible to assign the color of some γ-M2S3 materials to electronic transitions to the conduction band from (i) the valence band (La2S3), (ii) the 4f level (Ce2S3) and (iii) the 4f and valence band (Nd2S3).