A LANTHANIDE LANTHOLOGY (.pdf) - Davidson Physics

PHOSPHATESThe LnPO 4 compounds can be prepared [2], for example, by :• precipitation from aqueous solution,e.g. La 3+ + H 3 PO 4 => LaPO 4 .½H 2 O• reaction of phosphoric acid, or ammonium phosphates,with Ln salts such as the oxide or carbonate.The exothermic reactions should be controlled by slurrying the oxide orsalt in water. The more complex phosphates, e.g. metaphosphates, can bemade by the solid state reaction of the lanthanide salt and a phosphatesource mixed in appropriate ratios.In common with all Ln minerals, Monazite and Xenotime, in theirnaturally occurring forms, contain a "natural ratio" of many of the Lnelements, as well as other cations, and are not to be envisaged as purecompounds of any individual lanthanide. Monazite contains mainly lightlanthanides whereas Xenotime is a source for Yttrium and heavy Ln's.The monazite and xenotime structures have exceptional thermalstabilities and, probably, single-phase behavior up totheir melting points[3] and indeed this, combined with a limited chemicalreactivity, accounts for the difficulty of processing phosphate ores. (Lnphosphates have been suggested as hosts for the long-term storage ofradio-active waste.)The more complex phosphates on heating decompose to theorthophosphate, e.g. :LnPO4melting points°CMonazite-typeLa 2072Ce 2045Pr 1938Nd 1975Sm 1916Xenotime-typeY 1995Er 1896• La 5 PO 14 => La(PO 3 ) 3 => LaPO 4The doped lanthanum salt, (La,Ce,Tb)PO 4 , is now widely used as the preferredgreen-emitting phosphor for energy-efficient fluorescent lighting[4]. Ln phosphates have also beenproposed as scintillators and laser hosts.[2] Syntheses of Rare-Earth Orthophosphates, Y.Hikichi et al., Bull.Chem.Soc.Jpn., 1978, 51(12), 3645[3] Melting Temperatures of Monazite and Xenotime, Y. Hikichi and Ts. Nomura, J.Am. Ceram. Soc., 1987,70(10), C-252[4] Phosphors based on Rare-Earths, a New Era in Fluorescent Lighting, B.M.J.Smets, Mat. Chem. Phys.,1987, 16, 28329