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 A. The substantial width and marked asymmetry of this peak indicates that the minimum approach of Sm3+ ions could be as close as 4 A. 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, nSm(O), of 7, with a mean Sm-O bond length of 2.375(5) A whilst the PO4 tetrahedra have a mean P-O bond length of 1.538(2) A. 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 Sm - Sm separations