A LANTHANIDE LANTHOLOGY (.pdf) - Davidson Physics

SULFIDESFor Ln 2 S 3 systems, with Ln 3+ , several polymorphs are possible and, although the possibleexistence of products such as Ln 10 S 14 O can confuse analysis, the various structures have beencharacterized.[1] In general, in lanthanide sulfides, tetravalent Ln ions are not found (unlike theoxides) whereas divalent Ln, unstable in oxides, is more stable in sulfide systems ; the sulfideanion is less electronegative than its oxide counterpart. Polysulfides and more complexcompositions are possible.Non-stoichiometric compositions readily form and, in particular, materials can have acomposition range from Ln 2 S 3 through to Ln 3 S 4 , the single phase Th 3 P 4 structure being maintainedthroughout the range. (The generic formula is Ln 3-x [ ] x S 4 where [ ] is a vacancy site and x variesfrom 0 to 0.333.) The one extreme, Ln 2 S 3 , is an insulator, intermediates are semi-conducting, andthe other extreme, Ln 3 S 4 , is metallic [2]. (This conductivity behavior underlies the sulfide'spotential in high temperature thermo-electric conversion, heat to electricity, for power sources inspace.[3]) The cerium sulfides that have been characterized include not only the compound Ce 2 S 3 ,expected for Ce 3+ and S 2- , but also a black Ce 3 S 4 and CeS.CeS[4], bronze in color with a metallic lustre, adopts the NaCl structure with Ce 3+ , S 2- andone electron in a conduction band. This sulfide has a high electrical conductivity (in the metallicrange), a high thermal conductivity, a high melting point (≈2715 K) plus good thermal shockresistance amd, in addition, it is machineable like a metal.The red Ce 2 S 3 is comparable in color to cadmium sulfide derivatives and it has beenproposed as a pigment to replace those cadmium derivatives[5]. The composition of the ceriumsulfides can be fine-tuned, like the cadmium compounds to optimize coloring power etc.[1] Sulfides, Selenides and Tellurides, J.Flahaut, in "Handbook on the Physics and Chemistry of Rare Earths",ed. K.A.Gschneidner et al., Vol.4, 1 (Chap. 31), publ. North-Holland 1979[2] High-Temperature Thermo-dynamic Properties of Alpha and Gamma Lanthanum Sesquisulfides andRelated Compounds, T.Amano et al., J.Appl. Phys., 1986, 59(10), 3437[3] Thermoelectric Efficiency of Rare Earth Sesquisulfides, S.M.Taher and J.B.Gruber, Mat.Res.Bull., 1981,16, 1407 : Thermoelectric Properties of Lanthanum Sulfide, C.Wood et al., J.Appl.Phys., 1985, 58(4), 1542[4] Synthesis of CeS and Interactions with Molten Metals, O.H.Krikorian and P.G.Curtis, High Temp. HighPress., 1988, 20, 9[5] Compositions based on Rare Earth Sulfides, their Manufacture, Pigments containing Sulfides, and Use ofthe Pigments, T.Chopin et al., Eur.Patent Appl., 545746, 9 June 199344