COMPARATIVE-STUDY OF SOME RARE-EARTH SULFIDES - DOPED GAMMA-[A]M(2)S(3) (M=LA, CE AND ND, A=NA, K AND CA) AND UNDOPED GAMMA-M(2)S(3) (M=LA,CE AND ND)

The crystal structure determination of gamma-M(2)S(3) compounds (M = L a, Ce, and Nd) has been carried out for the first time from single cry stals obtained through high-temperature melting under sulfur pressure. The three phase structures do not depart from the cubic Th3P4 structu ral type, with a statistical filling of the dodecahedral sites by the cations. The gamma-Na0.5Ce2.5S4-doped phase structure has also been de termined from a powder neutron diffraction study. Na+ was found to be located at the dodecahedral site, in agreement with the composition li mit of Na/Ce=0.20 as determined by cell parameter variation versus com position. A powder X-ray diffraction study of the potassium- and calci um-doped derivatives (gamma-K0.46C2.54S4 and gamma-Ca0.89Ce2.07S4 conf irmed the results obtained for the sodium-doped phase. In no case, at least in the phases studied, does the alkali or alkaline earth metal o ccupy the inter-dodecahedral tetrahedral sites. The electronic band st ructures of Ce2S3 and of Ce3-xS4 (0<x<1/3) indicate an insulating beha vior for the former compound and a metallic behavior for the latter, a ssuming in this case a rigid band model. In Ce3S4, the electronic cond uctivity takes place along the Ce-Ce bonds. No S-S bonding was found i n the two binaries. It seems possible to assign the color of some gamm a-M(2)S(3) 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).