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

Abstract Three new crystal structures, isotypic with β-Zr 2 O(PO 4 ) 2 , have been resolved by the Rietveld method. All crystallize with an orthorhombic cell (S.G.: Cmca ) with a  = 7.1393(2) A, b  = 9.2641(2) A, c  = 12.5262(4) A, V  = 828.46(4) A 3 and Z  = 8 for Th(OH)PO 4 ; a  = 7.0100(2) A, b  = 9.1200(2) A, c  = 12.3665(3) A, V  = 790.60(4) A 3 and Z  = 8 for U(OH)PO 4 ; a  = 7.1691(3) A, b  = 9.2388(4) A, c  = 12.8204(7) A, V  = 849.15(7) A 3 and Z  = 4 for Th 2 O(PO 4 ) 2 . By heating, the M(OH)PO 4 (M = Th, U) compounds condense topotactically into M 2 O(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 2 O(PO 4 ) 2 compared to that of U 2 O(PO 4 ) 2 seems to result from this unusual environment for tetravalent thorium.