Multiple rare-earth ion environments in amorphous(Gd2O3)0.230(P2O5)0.770revealed by gadoliniumK-edge anomalous x-ray scattering

A Gd $K$-edge anomalous x-ray scattering (AXS) study is performed on the rare-earth $(R)$ phosphate glass, ${(\mathrm{G}{\mathrm{d}}₂{\mathrm{O}}₃)}_0.230{({\mathrm{P}}₂{\mathrm{O}}₅)}_0.770$, in order to determine $\mathrm{Gd}\ensuremath{\cdots}\mathrm{Gd}$ separations in its local structure. The minimum rare-earth separation is of particular interest given that the optical properties of these glasses can quench when rare-earth ions become too close to each other. To this end, a weak $\mathrm{Gd}\ensuremath{\cdots}\mathrm{Gd}$ pairwise correlation is located at $4.2(1)\phantom{\rule{0.16em}{0ex}}\AA{}$, which is representative of a metaphosphate $R\ensuremath{\cdots}R$ separation. More intense first-neighbor $\mathrm{Gd}\ensuremath{\cdots}\mathrm{Gd}$ pairwise correlations are found at the larger radial distributions, 4.8(1), 5.1(1), and $5.4(1)\phantom{\rule{0.16em}{0ex}}\AA{}$. These reflect a mixed ultraphosphate and metaphosphate structural character, respectively. A second-neighbor $\mathrm{Gd}\ensuremath{\cdots}\mathrm{Gd}$ pairwise correlation lies at $6.6(1)\phantom{\rule{0.16em}{0ex}}\AA{}$ which is indicative of metaphosphate structures. Meta- and ultraphosphate classifications are made by comparing the $R\ensuremath{\cdots}R$ separations against those of rare-earth phosphate crystal structures, $R{(\mathrm{P}{\mathrm{O}}₃)}₃$ and $R{\mathrm{P}}₅{\mathrm{O}}₁₄$, respectively, or difference pair-distribution function $(\mathrm{\ensuremath{\Delta}}\mathrm{PDF})$ features determined on similar glasses using difference neutron-scattering methods. The local structure of this glass is therefore found to display multiple rare-earth ion environments, presumably because its composition lies between these two stoichiometric formulae. These $\mathrm{Gd}\ensuremath{\cdots}\mathrm{Gd}$ separations are well-resolved in $\mathrm{\ensuremath{\Delta}}\mathrm{PDFs}$ that represent the AXS signal. Indeed, the spatial resolution is so good that it also enables the identification of $R\ensuremath{\cdots}X\phantom{\rule{4pt}{0ex}}(X=R$, P, O) pairwise correlations up to $r\ensuremath{\sim}9\phantom{\rule{0.16em}{0ex}}\AA{}$; their average separations lie at $r\ensuremath{\sim}7.1(1)$, 7.6(1), 7.9(1), 8.4(1), and $8.7(1)\phantom{\rule{0.16em}{0ex}}\AA{}$. This is a report of a Gd $K$-edge AXS study on an amorphous material. Its demonstrated ability to characterize the local structure of a glass up to such a long range of $r$ heralds exciting prospects for AXS studies on other ternary noncrystalline materials. However, the technical challenge of such an experiment should not be underestimated, as is highlighted in this work where probing AXS signal near the Gd $K$ edge is found to produce inelastic x-ray scattering that precludes the normal AXS methods of data processing. Nonetheless, it is shown that AXS results are not only tractable but they also reveal local structure of rare-earth phosphate glasses that is important from a materials-centered perspective and which could not be obtained by other materials characterization methods.