An EXAFS investigation of rare-earth local environment in ultraphosphate glasses

The local structures around rare-earth ions in rare-earth ultraphosphate glasses (nominally 15RE2O3–85P2O5, mol%, RE = Nd, Sm, Gd, Ho, and Er) and metaphosphate glasses (nominally 25RE2O3–75P2O5, mol%, RE = Nd, Gd, and Er) have been studied by extended X-ray absorption fine structure spectroscopy (EXAFS) at the respective rare-earth LIII edges. The RE–O distance decreases with increasing atomic number of the rare-earth ion for both ultra and metaphosphate glasses. On average, RE–O distances for metaphosphate glasses are shorter than those for ultraphosphate glasses of comparable compositions. There are 7–8 oxygen nearest neighbors in the first co-ordination sphere around rare-earth ions in ultraphosphate glasses and ∼6 oxygens in the first co-ordination sphere around rare-earth ions in metaphosphate glasses. There is no systematic dependence of the RE–O co-ordination number on the atomic number of the rare-earth ions in either compositional series. The second co-ordination shell around rare-earth ions in ultraphosphate glasses consists of phosphorus ions for all but the samarium ultraphosphate glass, with a RE–P distance between 3.53 and 3.85 Å and co-ordination numbers between 3 and 5. No RE–RE correlations were found within a distance of 4.5 Å.