STRUCTURAL STUDY OF THE RHOMBOHEDRAL FLUORITE-RELATED RIII PHASE U1-YLAYO2-X, SS-THAN-OR-EQUAL-TO-Y-LESS-THAN-OR-EQUAL-TO-0.67()

The structural relationships between the R(III) phase, existing in the UO3-UO2-LaO1.5 system for the compositional range U1-yLayO2+/-x(0.56 less-than-or-equal-to y less-than-or-equal-to 0.67), and the fluorite structure have been definitely established by means of electron diffra ction. The determination of the structure has been carried out using b oth X-ray and neutron powder diffraction. The R(III) phase constitutes a partially cation-ordered fluorite-related structure, where La and U /La layers alternate along [111]C. The rhombohedral lattice, in its he xagonal setting, is related to the cubic fluorite substructure by the relations a(H) = 1/2a(C) - 1/2b(C), b(H) = 1/2b(C) - 1/2C(C), C(H) = 2 (a(C) + b(C) + c(C)). The space group (R3mBAR), cell parameters a(H) 3 .9 angstrom and C(H) almost-equal-to 18.9 angstrom, and the refined st ructure show that the R(III) phase can be considered as isomorphous to the CaUO4 structure. (C) 1994 Academic Press, Inc.