X-RAY ABSORPTION SPECTROSCOPY INVESTIGATION OF THE Ta SITE IN ALPHA-RECOIL DAMAGED NATURAL PYROCHLORES.

X-ray absorption spectroscopy has been used to investigate the Ta L//I//I//I and L//I absorption edges of metamict and annealed pyrochlore minerals A//1// minus //2B//2O//6Y//0// minus //1. The L//I//I//I edge absorption spectra exhibit a pronounced doublet in the primary absorption resonance for the annealed samples which have the pyrochlore structure. The doublet in the L//I//I//I edge resonance becomes less pronounced and for some samples merges into a singlet for samples in the unannealed, fully damaged (metamict) state. The L//I//I//I edge resonance of the damaged samples closely resembles Ta//2O//5 or disordered manganotantalite (MnTaNbO//6) while the L//I//I//I edge resonance for the annealed samples most closely resembles the synthetic pyrochlore KTaWO//6 multiplied by (times) H//2O. These results are supported by Ta L//I edge x-ray absorption data. Fourier transforms of the Ta L//I//I//I edge x-ray absorption fine structure (EXAFS) show first neighbor Ta-O bond lengths of 1. 96 plus or minus 0. 05 A for samples in the annealed and metamict state. The complex structure of Ta//2O//5 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.