Structure of ultraphosphate glasses with small rare-earth ions by X-ray diffraction

Rare-earth ultraphosphate glasses with nominal R2O3 fractions of 10 and 15 mol% and small ionic radius (large atomic number) R3+ ions (R = Tb, Tm, Lu) are measured by X-ray diffraction at a synchrotron with photons of 119 keV (maximum scattering vector 260 nm− 1). The total correlation functions T(r) show well-resolved R-O and O-O first-neighbor peaks. In contrast to all ultraphosphate RP5O14 crystals and the ultraphosphate glasses of larger R3+ ions, where RO8 polyhedra (mean R-O coordination numbers of ~ 8 for the glasses) are observed, the R3+ ions in glasses with R = Lu, Tm, Tb have mean R-O coordination numbers of ~ 7.5. The R-O first-neighbor peaks extracted from the T(r) functions are compared with those obtained from atomic coordinates of related RP5O14 and RP3O9 crystals. The R-O distances of the ultraphosphate glasses studied are found to fall between those of the two crystals but with tails to the side of longer bonds.