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.