AN EXAFS STUDY OF RARE-EARTH PHOSPHATE-GLASSES IN THE VICINITY OF THEMETAPHOSPHATE COMPOSITION

A study of rare-earth phosphate glasses, in the vicinity of the metaph osphate composition, has been undertaken at different temperatures usi ng the extended X-ray absorption fine structure (EXAFS) facility at th e Synchrotron Radiation Source, Daresbury Laboratory, UK. The metaphos phate-like glasses examined contained the rare-earth elements La, Sm. Eu and Gd as R3+ ions. The experiments were carried out at room temper ature, 145 and 79 K. The data show that the first shell surrounding ra re-earth ions contains only oxygen atoms at a mean distance of similar to 2.2-2.4 Angstrom. The 'Iantfianide contraction' is clearly observe d, 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 co rrelation shell was found associated with phosphorus atoms around the central rare-earth atom (similar to 3.5 Angstrom), and another rare-ea rth:oxygen correlation was also identified (similar to 4 Angstrom). St atic 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 studie d. We conclude that the anomalous bulk properties are associated with either subtle structural features or primarily due to rare-earth ion I nteractions. (C) 1998 Elsevier Science B.V. All rights reserved.