ON THE DIMORPHISM AND CRYSTAL CHEMICAL-FEATURES OF THE PLUTONIUM(IV) PHOSPHATE NAPU2(PO4)3

The x-ray powder patterns of two crystalline modifications of sodium Pu(IV)phosphate that are formed at high temperatures in the PuO2-NaH2PO4-B2O3 system are interpreted. One of the NaPu2(PO4)3 modifications is monoclinic with constants a = 17.14, b = 6.6.5, c = 7.90 angstrom, beta = 101. 12-degrees, and Z = 4. It is isostructural with the analogous Th, U(IV), and Np(IV) compounds. The other modification crystallizes in the trigonal system with constants a = 9.09, c = 23.69 angstrom, and Z = 6 It has the structure of alkali Zr and Hf double phosphates. A comparison of the crystal structures of the two modifications reveals substantial differences in the atomic structure. The reasons leading to the morphotropic transition in the series of Th, U, Np, and Pu phosphates and the nature of the dimorphism of NaPu2(PO4)3 are discussed.