An EXAFS study of rare-earth phosphate glasses in the vicinity of the metaphosphate composition

A study of rare-earth phosphate glasses, in the vicinity of the metaphosphate composition, has been undertaken at different temperatures using the extended X-ray absorption fine structure (EXAFS) facility at the Synchrotron Radiation Source, Daresbury Laboratory, UK. The metaphosphate-like glasses examined contained the rare-earth elements La, Sm. Eu and Gd as R3+ ions. The experiments were carried out at room temperature, 145 and 79 K. The data show that the first shell surrounding rare-earth ions contains only oxygen atoms at a mean distance of similar to 2.2-2.4 Angstrom. The 'Iantfianide contraction' is clearly observed, i.e., the R-O distance decreases with increasing atomic number. The observed R(-O) coordination numbers are in the range 5-7. A second correlation shell was found associated with phosphorus atoms around the central rare-earth atom (similar to 3.5 Angstrom), and another rare-earth:oxygen correlation was also identified (similar to 4 Angstrom). Static disorder dominates the Debye-Waller term, but thermal disorder is not negligible. Within the accuracy of the EXAFS data, no significant structural variations were observed over the temperature range studied. We conclude that the anomalous bulk properties are associated with either subtle structural features or primarily due to rare-earth ion Interactions. (C) 1998 Elsevier Science B.V. All rights reserved.