A rare-earth K-edge EXAFS study of rare-earth phosphate glasses,(R2O3)x(P2O5)1-x, x = 0.187-0.239, R = La, Nd, Sm, Eu, Gd,Dy, Er

Abstract

A rare-earth K-edge extended x-ray absorption fine structure (EXAFS) study ofrare-earth phosphate glasses, (R2O3)x(P2O5)1-x,x = 0.187-0.239, R = La, Nd, Sm, Eu, Gd, Dy, Er, ispresented. The structures of these materials were investigated as a functionof (a) rare-earth atomic number and (b) temperature, and represent some of thefirst rare-earth K-edge EXAFS studies on a series of lanthanide-basedmaterials. Results corroborate findings from complementary x-ray and neutrondiffraction and magic-angle-spinning (MAS) NMR experiments, and in addition,they provide a unique insight into the nature of the static disorder of theR-O correlations and of the neighbouring phosphate groups. The effects ofmultiple-scattering contributions are also discussed within this context. Thevariable temperature measurements illustrate the exceptionally high level ofnetwork rigidity present in these materials. The results are also compared tothose obtained from an analogous rare-earth LIII-edge EXAFS(5.483-8.358 keV) study. Results show that the use of the much higherenergies of the rare-earth K-edge (38.925-57.486 keV) enable one to avoid thedouble-electron excitation problems that are associated with the rare-earthLIII-edge EXAFS in the dynamic range of interest. EXAFS fitting anddeconvolution simulations show that the large core hole lifetimes associatedwith the rare-earth K-edge do not significantly detract from the results. Thedeconvolution studies also corroborate our findings that the level of fittingto our data cannot realistically be expanded beyond the first R-O shell. Thislimitation exists despite the exceptional counting statistics of theexperiment and the highly uniform samples made possible by the ability to usemuch thicker samples at the higher energies compared to those used for the(higher absorption) rare-earth LIII-edge EXAFS studies.