Disordered Gd6UO12-δ with the cation antisite defects prepared by a combined mechanochemical−thermal method

Abstract

• The rhombohedral Gd 6 UO 12-δ single phase is synthesized for the first time via mechanochemical − thermal route. • The as-prepared uranate exhibits a remarkable degree of cation antisite disorder and a relatively high concentration of oxygen vacancies . • The XRD simulations reveal a decrease of the intensity of superlattice reflections with increasing degree of cation antisite defects in Gd 6 UO 12-δ . • The nonequilibrium cation distribution in Gd 6 UO 12-δ does not influence the symmetrical geometry of polyhedra around the 3 a sites, whereas the octahedra around the 18 f sites are found to be distorted. • The six-fold coordination of cations at 18 f sites is a consequence of a relatively large oxygen deficiency in the as-prepared material. The synthesis of the rhombohedral Gd 6 UO 12-δ is reported via mechanochemical processing of stoichiometric Gd 2 O 3 /UO 2 mixtures and their subsequent annealing. Rietveld refinement of XRD data reveals that the as-prepared material exhibits a remarkable degree of cation antisite disorder and oxygen deficiency. The simulations of intensities of the selected XRD superlattice reflections are performed for limiting states of Gd 6 UO 12-δ with its most extreme degrees of the cation antisite disorder. On the basis of the estimated bond lengths it can be stated that distorted geometry of structural units in the material is a consequence of its relatively large oxygen deficiency. The cations in Gd 6 UO 12-δ prepared by mechanochemical − thermal route are distributed over the 3 a and 18 f sites of octahedral coordination with the symmetrical and distorted geometry, respectively.