Rare-earth phosphate glasses exhibit exotic optical and magnetic properties, which are dictated by their structure, principally the nature of the closest R R separation. Conventional structural characterization tools have enabled the comprehensive determination of the radial distribution from the rare-earth ion out to 4 Å. At or beyond this distance, however, lies the crucially important minimum R 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 R R separations in rare-earth phosphate glasses, (R2O3)x(P2O5)1−x, in the range 0.167 < x < 0.25, both from experimental and computational means. Coupled with tabulations of all known minimum R R separations in analogous meta- and ultra-phosphate crystal structures, a collective comparison of all of these R 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 R R distance lies at 4.0 Å whilst a second, much broader, nearest-neighbour R R correlations is centered between 6.0–6.4 Å. However, glasses that contain rare-earth ions at least as heavy as holmium are likely to exhibit a very different structure, with a minimum R R separation of 5.4–5.6 Å. 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 R R distribution significantly. Overall, there is excellent agreement between experiment and theory
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