An extended x-ray absorption fine structure study of rare-earth phosphate glasses near the metaphosphate composition

Abstract

A variable-temperature (79, 145, and 293 K) extended x-ray absorption fine structure study, using rare-earth L III absorption edges, is reported for phosphate glasses doped with rare-earth elements (R, where R ‫ס‬ La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er) with compositions close to metaphosphate, R(PO 3 ) 3 . The results yield nearest-neighbor R-O distances that demonstrate the lanthanide contraction in a glassy matrix and an R-O coordination intermediate between 6 and 7 for rare-earth ions with smaller atomic number (Z) and 6 for rare-earth ions with larger Z. Thermal parameters show no significant changes in R-O distances or coordination numbers between 293 and 79 K. There is evidence of an R-P correlation between 3.3 and 3.6 Å and the beginning of a second R-O correlation at approximately 4 Å. No R-R correlations up to a distance of approximately 4 Å were observed.

Vibrational spectra of R(PO3)3 metaphosphates (R = Ga, In, Y, Sm, Gd, Dy)

“…Glasses containing La-Nd, however, avoid that bonding scheme, as evidenced by recent neutron diffraction and EXAFS studies. 8,10,12 The preference of the majority of R(PO 3 ) 3 structures for twofold bonding of NBOs is primarily due to the transfer of covalent bonding in the direction of the R-O bond. These structures involve cations with large charges in order to maintain reasonable bond orders on the oxygen atoms.…”

A study of low energy magnetic excitations in terbium metaphosphate glass

“…To isolate the form of the excess magnetic contribution to the terbium glass spectra, the phonon spectrum must be modelled and subtracted from the raw data. The most reliable and straightforward way to achieve this is to assume that the vibrational components of all the rare earth metaphosphate glasses are identical; this is a reasonable postulate as all the lanthanides are based on a xenon core, and are situated in the same host material [5]. Therefore the phonon contribution can be measured directly from the inelastic neutron scattering spectrum of lanthanum metaphosphate glass, also reported in Fig.…”

“…The trivalent Tb 3þ ion has J ¼ 6 in its ground-state multiplets and an energy-level diagram that is somewhat more complex than for Pr 3þ . The local structure of terbium metaphosphate glass has already been examined using the complementary probs of X-ray diffraction and EXAFS [4,5], although details of the longer range structure are lacking. There is a wide consensus, however, that the network of phosphate glasses is formed from chains, rings and branched polymeric units of interconnected PO 4 tetrahedra, the metallic cations being hosted in the interstices of the skeletal structure.…”

Structure of rare-earth phosphate glasses by X-ray and neutron diffraction

“…Since it is well known that the ionic radii of the lanthanides decrease with the increasing atomic numbers, studies of such series attract special interest. The corresponding changes were investigated on series of metaphosphate glasses with R ranging from La to Er [12][13][14][15]. The expected decrease of R-O distances was found.…”

“…The expected decrease of R-O distances was found. From comparisons with tabulated ionic radii [16][17][18] given for different coordination environments, it was concluded [13][14][15] that coordination numbers (N RO ) should decrease from $7 to $6 if R varies from La to Er. However, the uncertainties of the N RO obtained for the corresponding diffraction peaks in many of these earlier studies were too large to confirm this coordination number decrease.…”