Ti-site geometry in metamict, annealed and synthetic complex TiNbTa oxides by X-ray absorption spectroscopy

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopy has been used to determine the nearest neighbor environment of Ti in metamict AB ₂O ₆-type complex oxides (euxenite and aeschynite structure types) in order to evaluate the effect of alpha-recoil damage of these structures. Comparison of the EXAFS/XANES data for metamict samples with data for annealed and crystalline samples suggests minor changes in the first coordination sphere, TiO, (a slight decrease in coordination number and bond lengths, and increased distortion of the octahedral coordination polyhedron), but major disruption of the second coordination sphere, TiTi, for the material in the metamict state. These data suggest a mechanism for the transition from the crystalline to the metamict state in which tilting of cation coordination polyhedra is the principal effect of damage caused by alpha-recoil events.