Crystal structures of Th(OH)PO4, U(OH)PO4 and Th2O(PO4)2.: Condensation mechanism of MIV(OH)PO4 (M = Th, U) into M2O(PO4)2

Three new crystal structures, isotypic with beta-Zr2OWO4)(2), have been resolved by the Rietveld method. All crystallize with an orthorhombic cell (S.G.: Cmca) with a = 7.1393(2) angstrom, b = 9.2641(2) angstrom, c = 12.5262(4) angstrom, V= 828.46(4) angstrom(3) and Z = 8 for Th(OH)PO4; a = 7.0100(2) angstrom, b=9.1200(2)angstrom, c=12.3665(3)angstrom, V=790.60(4)angstrom(3) and Z=8 for U(OH)PO4; a=7.1691(3)angstrom, b=9.2388(4)angstrom, c=12.8204(7)angstrom, V = 849.15(7) angstrom(3) 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. (c) 2007 Elsevier Masson SAS. All rights reserved.