The local environment of trivalent lanthanide ions in sodium silicate glasses: A neutron diffraction study using isotopic substitution

Neutron diffraction data on lanthanide-bearing sodium silicate glasses were collected using the glass, liquid and amorphous materials diffractometer (GLAD) at the intense pulsed neutron source (IPNS), Argonne. Measurements were made on four glass samples; a sodium silicate base glass with no added lanthanide, a sample with 6 mol% La₂O₃ added to the base composition, and two isotopically-distinct samples containing 6 mol% Dy₂O₃. Of the Dy-bearing samples, one contained natural enrichment Dy, and the other a 'null scattering' mixture of natural and ¹⁶²Dy isotopes. The first-order isotope difference of the scattering from the Dy-bearing samples revealed a Dy-O nearest-neighbor distance of 2.3 Å and a mean coordination number of 5.9 oxygen atoms around each dysprosium ion; the latter is in good agreement with the coordination number derived from bond-valence theory. The results for the La-bearing glass were also consistent with a sixfold coordination of oxygen atoms around the rare earth. The diffraction data are used in combination with Reverse Monte Carlo modeling techniques to interpreter the structural role of the rare earth ions. From these models, it is apparent that there is competition between monovalent sodium ions and the rare earth ions for the same non-bridging oxygen atoms. As a result the addition of rare earth ions appears to cause disruption of the Na-rich percolation domains characteristic of the sodium silicate base glass. These neutron results are consistent with the formation of a specific lanthanide Q³ as suggested by NMR and Raman spectroscopy, but offer no direct evidence for the formation of 'oxide-like' lanthanide clusters.