Structure of lanthanum and cerium phosphate glasses by the method of isomorphic substitution in neutron diffraction

Neutron diffraction was used to measure the total structure factors for several rare-earth ion R³⁺ (La³⁺ or Ce³⁺) phosphate glasses with composition close to RAl0.35P3.24O10.12. By assuming isomorphic structures, difference function methods were employed to separate, essentially, those correlations involving R³⁺ from the remainder. A self-consistent model of the glass structure was thereby developed in which the Al correlations were taken into explicit account. The glass network was found to be made from interlinked PO₄ tetrahedra having 2.2(1) terminal oxygen atoms, OT, at 1.51(1) {}, and 1.8(1) bridging oxygen atoms, OB, at 1.60(1) {}. Rare-earth cations bonded to an average of 7.5(2) OT nearest neighbors in a broad and asymmetric distribution. The Al³⁺ ion acted as a network modifier and formed OT-Al-OT linkages that helped strengthen the glass. The connectivity of the R-centered coordination polyhedra was quantified in terms of a parameter fₛ and used to develop a model for the dependence on composition of the Al-OT coordination number in R-Al-P-O glasses. By using recent ¹⁷Al nuclear-magnetic-resonance data, it was shown that this connectivity decreases monotonically with increasing Al content. The chemical durability of the glasses appeared to be at a maximum when the connectivity of the R-centered coordination polyhedra was at a minimum. The relation of fₛ to the glass transition temperature, Tg, was discussed.