An X-ray Absorption Spectroscopy Investigation of the Ta Site in Alpha-Recoil Damaged Natural Pyrochlores

Abstract

AbstractX-ray absorption spectroscopy has been used to investigate the Ta LIII and LI absorption edges of metamict and annealed pyrochlore minerals A1-2B2O6Y0-1. The LIII edge absorption spectra exhibit a pronounced doublet in primary absorption resonance for the annealed samples which have the pyrochlore structure. The doublet in the LIII edge resonance becomes less pronounced and for some samples even merges Into a singlet for the samples in the unannealed, fully damaged (metamict) state. The LIII edge resonance of the damaged samples closely resembles Ta205 or disordered manganotantalite (MnTaNbO6 ), while the LIII edge resonance for the annealed samples most closely resembles the synthetic pyrochlore KTaWO6·H20. These results are supported by the Ta LIIIedge x-ray absorption data. Fourier transforms of the Ta LIII edge x-ray absorption fine structure (EXAFS) show first neighbor Ta-0 bond lengths of 1.96 ± 0.05 A for the samples in the annealed and metamict state.The complex structure of Ta205 is used as a model for the Ta-containing oxygen polyhedra in metamict pyrochlores, as it embodies some of the topologic features consistent with the reduction of long-range periodicity with increasing alpha-decay dose: 1) greater distortion of the primary coordination shell around Ta, 2) slightly increased mean second nearest-neighbor <Ta…X> distances, and 3) a wider distribution of second nearest-neighbor distances. These modifications are incorporated in a large unit cell with a b translation of 40 A.