Direct observation of the R•••R separation in rare-earth phosphate glasses

Rare-earth phosphate glasses exhibit exotic optical and magnetic properties, which are dictated by their structure, principally the nature of the closest (RR)-R-... separation. Conventional structural characterization tools have enabled the comprehensive determination of the radial distribution from the rare-earth ion out to similar to 4 angstrom. At or beyond this distance, however, lies the crucially important minimum (RR)-R-... separation. This study illustrates three types of non-conventional diffraction experiments that overcome the radial limitations of regular structural probes. The results from such investigations are compiled with all other known estimates of (RR)-R-... separations in rare-earth phosphate glasses, (R2O3),(P2O5)(1-x), in the range 0. 167 < x < 0.25, both from experimental and computational means. Coupled with tabulations of all known minimum (RR)-R-... separations in analogous meta- and ultra-phosphate crystal structures, a collective comparison of all of these (RR)-R-... separations permits a hypothesis by which all results are consistent. That hypothesis indicates that where rare-earth phosphate glasses are near meta-phosphate, their minimum (RR)-R-... distance lies at similar to 4.0 angstrom whilst a second, much broader, nearest-neighbour (RR)-R-... correlations is centered between 6.0-6.4 angstrom. However, glasses that contain rare-earth ions at least as heavy as holmium are likely to exhibit a very different structure, with a minimum (RR)-R-... separation of 5.4-5.6 angstrom. Structures that lie intermediate between the ultra-phosphate and meta-phosphate composition may display mixed characteristics of these two extremes. If present in high levels, aluminium contamination arising from the synthetic process may also affect the (RR)-R-... distribution significantly. Overall, there is excellent agreement between experiment and theory. (c) 2007 Elsevier B.V. All rights reserved.