Neutronenbeugungsuntersuchungen an (SE 0·5U 0·5)O 2 (SE = Y, La, Nd, Ho, und Lu)

Neutron diffraction studies of (RE _0.5U _0.5)O _2.00 (RE = Y, La, Nd, Ho, Lu) and (La _0.67U _0.33)O _2.00 ternary oxides have proved that these ternary oxides crystallize in the fluorite structure. The oxygen atoms are located on the normal anion sites of the fluorite structure, thus excluding a covalent UO ⁺₂ group. In these ternary oxides the lanthanide atoms as well as U(V) and U(VI) have a nondistorted cubic environment (coordination No. 8), for U(VI) such a coordination is reported for the first time. The mean vibration amplitude of the oxygen atoms ( overlineu ²_{S O}) ^{1}/{2} and the lattice constant increases with the mean ionic radius of the metal atoms in the (RE _0.5U _0.5)O _2.00 phases.