A LANTHANIDE LANTHOLOGY (.pdf) - Davidson Physics

SULFATESLanthanide sulfates can be prepared by addition of hydroxide, carbonate or other salts tosulfuric acid. They can form, on crystallization into a solid phase, a wide range of hydrates,Ln 2 (SO 4 ) 3 .nH 2 O,[1] with n varying from 1 to ≈8, with the octahydrate the species often obtainedunder equilibrium conditions. Simple lanthanide sulfates are moderately soluble in water andacid[2], from La≈40gm/l to Lu ≈400 gm/l (at 20 °C), with - for most Ln's - solubility decreasingwith increasing temperature.Many Ln(III) double sulfates, with alkali metal and ammonium cations such asLn 2 (SO 4 ) 3 .Na 2 SO 4 .2H 2 O, are known with varying degrees of aqueous solubility. These so-called"pink" salts have been explored - because their solubility changes as a function of the Ln - asintermediates in processes to effect a rough separation between lights and heavies. Other salts suchas acid sulfates, e.g Ce 2 (SO 4 ) 3 .mH 2 SO 4 .nH 2 O, and hydroxysulfates, La(OH)SO 4 , have beencharacterized. Much published knowledge in this area arose from the initial atomic energy projectwork on rare earth separation. Sulfuric acid processing can be used for the processing of bastnasiteand monazite.Cerium(IV), ceric, forms a stable sulfate, Ce(SO 4 ) 2 , that is strongly acidic in solution due tohydrolysis. The double salt, ceric ammonium sulfate, is a stable orange compound with Ce(IV)present in the anion, not the cation. The crystal structure shows this, and related compounds, to betruly sulfatocerates, e.g. [(NH 4 ) 2 ] [Ce(SO 4 ) 3 ]; these complex anions are very stable in aqueoussystems. Europium(II) forms a nearly insoluble salt, EuSO 4 , that provides a chemical step for thepurification of europium.Upon heating the Ln sulfates, water of hydration is lost but discrete intermediate hydrates arenot necessarily seen. The anhydrous sulfate is formed at temperatures in the range 250 to 400 °C.At higher temperatures, approaching 850 °C, an oxysulfate, is produced that is stable up to ≈1200°C, before ultimately converting to the oxide. The precise temperatures at which the intermediatesappear vary from lanthanide to lanthanide.Ln 2 (SO 4 ) 3 .nH 2 O => Ln 2 (SO 4 ) 3 => Ln 2 O 2 SO 4 => Ln 2 O 3The oxysulfate phase, Ln 2 O 2 SO 4 , is known for all lanthanides, all with the same structurebased on layers of [LnO] n n+ composition separated by [SO 4 ] 2- groups.[1] Properties of various Cerium Sulfates, O.V.Govorukhina et al., Russ,J.Inorg.Chem., 1984,29(6),822[2] Solubility of some Lanthanide Sulfates in Polycomponent Systems containing H 2SO 4, D.S.Todorovsky etal., Monats. Chemie, 1993, 124, 67343