The structure of the rare-earth phosphate glass(Sm₂O₃)_0.205(P₂O₅)_0.795 studied by anomalous dispersion neutron diffraction

The role of the Sm³⁺ ions in the structure of vitreous Sm₂O³•4P₂O₅
has been investigated using the neutron diffraction anomalous
dispersion technique, which employs the wavelength dependence of the
real and imaginary parts of the neutron scattering length close to an
absorption resonance. The data described here represent the first
successful complete neutron anomalous dispersion study on an amorphous
material. This experimental methodology permits one to determine
exclusively the closest Sm• •• Sm separation. Knowledge of the R•••R
(R = rare-earth) pairwise correlation is key to understanding the
optical and magnetic properties of rare-earth phosphate glasses. The
anomalous difference correlation function, ΔT''(r), shows a dominant feature pertaining to a Sm•••Sm separation, centred at 4.8 Å. The substantial width and marked asymmetry of this peak indicates that the minimum approach of Sm³⁺ ions could be as close as 4 Å. Information on other pairwise correlations is also revealed via analysis of T (r) and ΔT (r) correlation functions: Sm³⁺ ions display an average co-ordination number, n _Sm(O), of 7, with a mean Sm–O bond length of 2.375(5) Å whilst the PO₄
tetrahedra have a mean P–O bond length of 1.538(2) Å. Second- and
third-neighbour correlations are also identified. These results
corroborate previous findings. Such consistency lends support to the
application of the anomalous dispersion technique to determine separations.