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1st International Symposium on Structure-Property Relationships in Solid State Materials, Bordeaux :
Crystal structures of Th(OH)PO_4, U(OH)PO_4 and Th_2 O(PO_4)_2. Condensation mechanism of M^IV (OH)PO_4 (M = Th, U) into M_2 O(PO_4)_2
N. Dacheux1, N. Clavier1, G. Wallez2, M. Quarton2

Three new crystal structures, isotypic with β-Zr2O(PO4)2, have been resolved by the Rietveld method. All crystallize with an orthorhombic cell (S.G.: Cmca) with a = 7.1393(2) Å, b = 9.2641(2) Å, c = 12.5262(4) Å, V = 828.46(4) Å3 and Z = 8 for Th(OH)PO4; a = 7.0100(2) Å, b = 9.1200(2) Å, c = 12.3665(3) Å, V = 790.60(4) Å3 and Z = 8 for U(OH)PO4; a = 7.1691(3) Å, b = 9.2388(4) Å, c = 12.8204(7) Å, V = 849.15(7) Å3 and Z = 4 for Th2O(PO4)2. By heating, the M(OH)PO4 (M = Th, U) compounds condense topotactically into M2O(PO4)2, with a change of the environment of the tetravalent cation that lowers from 8 to 7 oxygen atoms. The lower stability of Th2O(PO4)2 compared to that of U2O(PO4)2 seems to result from this unusual environment for tetravalent thorium.
1 :  IPNO - Institut de Physique Nucléaire d'Orsay
2 :  LCMCP (site Paris VI) - Laboratoire de Chimie de la Matière Condensée de Paris (site Paris VI)