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 RAl_0.30P_3.05O_9.62 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) Å and 1.68(9) bridging oxygen atoms, O_B, at 1.60(1) Å. The network modifying R³⁺ 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) Å. 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.