CRYSTAL STRUCTURES OF U(Ta,W)5O16 AND U(Ta,W)2O8 REVEALED BY HIGH-RESOLUTION TRANSMISSION ELECTRON MICROSCOPY

摘要

Two new phases, U(Ta,W)5O16 and U(Ta,W)2O8, were formed by solid state reaction of the binary oxides (U3O8, Ta2O5, and WO3) and Ta metal powder. They were characterized by high-resolution TEM and powder XRD techniques. The phases are orthorhombic with the following unit cell dimensions: U(Ta,W)5O16, a=10.052(1), b=7.3207(5), and c=7.9600(4) angstroms; and U(Ta,W)2O8, a=20.232(2), b=7.4520(4), and c=4.0652(2) angstroms. Both structures are built up of (Ta,W)O6-octahedra arranged as ReO3-type slabs two octahedra wide. In U(Ta,W)5O16 these slabs are connected by a thin lamella consisting of hexagonal UO8-bipyramids and (Ta,W)O6-octahedra alternating along both the b and c axes. EDS analysis indicated a composition of UTa1.5W3.5O16. The structure is related to UMo5O16, but with every other layer displaced through _b. The second phase, U(Ta,W)2O8, is isotypic with the orthorhombic modification of UMo2O8, in which the ReO3-type slabs are intergrown with lamellas of edge-sharing pentagonal UO7-bipyramids. The composition UTa0.5W1.5O8 was obtained by EDS analysis. Some defects in terms of three-octahedra-wide ReO3-type slabs have been observed in U(Ta,W)5O16 crystals. EDS analyses of such crystals showed a high-W and a low-Ta content. 24 refs.