Crystal structures of Th(OH)PO 4, U(OH)PO 4 and Th 2O(PO 4) 2. Condensation mechanism of M IV(OH)PO 4 (M = Th, U) into M 2O(PO 4) 2

Abstract

Three new crystal structures, isotypic with β-Zr 2O(PO 4) 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) Å 3 and Z = 8 for Th(OH)PO 4; a = 7.0100(2) Å, b = 9.1200(2) Å, c = 12.3665(3) Å, V = 790.60(4) Å 3 and Z = 8 for U(OH)PO 4; a = 7.1691(3) Å, b = 9.2388(4) Å, c = 12.8204(7) Å, V = 849.15(7) Å 3 and Z = 4 for Th 2O(PO 4) 2. By heating, the M(OH)PO 4 (M = Th, U) compounds condense topotactically into M 2O(PO 4) 2, with a change of the environment of the tetravalent cation that lowers from 8 to 7 oxygen atoms. The lower stability of Th 2O(PO 4) 2 compared to that of U 2O(PO 4) 2 seems to result from this unusual environment for tetravalent thorium.