Structure of dysprosium and holmium phosphate glasses by the method of isomorphic substitution in neutron diffraction

The relative distribution of rare-earth ions R3+, (Dy3+ or Ho3+) in the phosphate glass RAl0.30P3.05O9.62 was measured by employing the method of isomorphic substitution in neutron diffraction and, by taking the role of Al into explicit account, a self-consistent model of the glass structure was developed. The glass network is found to be made from corner sharing PO4 tetrahedra in which there are, on average, 2.32(9) terminal oxygen atoms, O-T, at 1.50(1) Angstrom and 1.68(9) bridging oxygen atoms, OB, at 1.60(1) Angstrom. The network modifying R 3, ions bind to an average of 6.7(1) O-T and are distributed such that 7.9(7) R-R nearest neighbours reside at 5.62(6) Angstrom. The Al3+ ion also has a network modifying role in which it helps to strengthen the glass through the formation of O-T-Al-O-T linkages. The connectivity of the R-centred coordination polyhedra in (M2O3)(x)(P2O5)(1-x) glasses, where M3+ denotes a network modifying cation (R3+ or Al3+), is quantified in terms of a parameter f(s). Methods for reducing the clustering of rare-earth ions in these materials are then discussed, based on a reduction of f(s) via the replacement of R3+ by Al3+ at fixed total modifier content or via a change of x to increase the number Of O-T available per network modifying M3+ cation.