The structure of the rare-earth phosphate glass(Sm2O3)0.205(P2O5)0.795 studied by anomalous dispersion neutron diffraction

Abstract

The role of the Sm3+ ions inthe structure of vitreous Sm2O3·4P2O5 has been investigated using the neutron diffraction anomalous dispersion technique, whichemploys the wavelength dependence of the real and imaginary parts of the neutronscattering length close to an absorption resonance. The data described here represent thefirst successful complete neutron anomalous dispersion study on an amorphous material.This experimental methodology permits one to determine exclusively the closest separation. Knowledge of the (R = rare-earth) pairwise correlation is key to understanding the optical and magnetic propertiesof rare-earth phosphate glasses. The anomalous difference correlation function,ΔT′′(r), shows a dominant feature pertaining to a separation, centred at 4.8 Å. The substantial width and markedasymmetry of this peak indicates that the minimum approach ofSm3+ ions could be as close as 4 Å. Information on other pairwise correlations is also revealed via analysis ofT(r) andΔT(r) correlationfunctions: Sm3+ ions display an average co-ordination number,nSm(O), of 7, with a mean Sm–O bond length of 2.375(5) Å whilst thePO4 tetrahedra have a mean P–O bond length of 1.538(2) Å. Second- and third-neighbourcorrelations are also identified. These results corroborate previous findings. Suchconsistency lends support to the application of the anomalous dispersion technique todetermine separations.