Bob Newport, the presenting author, is Emeritus Professor of Materials Physics. He has an abiding fascination with the atomic-scale structure of amorphous solids, and in the coherent development and use of a materials-centred methodology in their study. In ‘retirement’ he has greatly expanded his science-focused public engagement activities. The principle author, JMC, with whom the data presented herein was collected, holds positions at ISIS (Harwell Science & Innovation Campus) and the Argonne National Laboratory (USA) as well as at the University of Cambridge.
Rare-earth ion environments in amorphous (Gd2O3)0.230(P2O5)0.770 revealed by gadolinium K-edge anomalous X-ray scattering
Jacqueline M. Cole1, Alisha J. Cramer1, Sarvjit D. Shastri2, Karim T. Mukaddem1 and Robert J. Newport3*
1Cavendish Laboratory, Department of Physics, University of Cambridge,
J. J. Thomson Avenue, Cambridge, CB3 0HE, UK.
2Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA.
3School of Physical Sciences, Ingram Building, University of Kent, Canterbury, CT2 7NH, UK.
A Gd K-edge anomalous X-ray scattering (AXS) study is performed on the rare-earth (R) phosphate glass, (Gd2O3)0.230(P2O5)0.770, in order to determine Gd…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 Gd…Gd pairwise correlation is located at 4.2(1) Å which is representative of a meta-phosphate R…R separation. More intense first neighbor Gd…Gd pairwise correlations are found at the larger radial distributions, 4.8(1) Å, 5.1(1) Å and 5.4(1) Å. These reflect a mixed ultra-phosphate and meta-phosphate structural character, respectively. A second neighbor Gd…Gd pairwise correlation lies at 6.6(1) Å which is indicative of meta-phosphate structures. Meta- and ultra-phosphate classifications are made by comparing the R…R separations against those of rare-earth phosphate crystal structures, R(PO3)3 and RP5O14 respectively, or difference pair distribution function (ΔPDF) 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 Gd…Gd separations are well resolved in the ΔPDFs that represent the AXS signal. Indeed, the spatial resolution is so good that it also enables the identification of R…X (X = R, P, O) pairwise correlations up to r ~ 9 Å; their average separations lie at r ~ 7.1(1) Å, 7.6(1) Å 7.9(1) Å, 8.4(1) Å and 8.7(1) Å. This is the first 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 non-crystalline 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 resonant 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.