A neutron diffraction and Al-27 MQMAS NMR study of rare-earth phosphate glasses, (R2O3)(x)(P2O5)(1-x), x=0.187-0.263, R = Ce, Nd, Tb containing Al impurities

A neutron diffraction study of four rare-earth phosphate glasses of composi tion (R2O3)(x)(P2O5)(1-x), where x = 0.197, 0.235, 0.187, 0.263 and R = Ce, Ce, Nd, Tb respectively is presented. The structures of these materials we re investigated as a function of (a) rare-earth atomic number and (b) compo sition. The results show that samples containing the larger rare-earth ions (Ce3+ and Nd3+) are coordinated to seven oxygen atoms whereas the immediat e environment of Tb3+ ions is six coordinate. This implies that rare-earth clustering must be present in the samples containing larger rare-earth ions although no R...R correlations are directly observed. Terminal and bridgin g P-O correlations are resolved, existing in an approximately I:I ratio. Se cond-neighbour O(P)O separations are located with good accuracy and P(O)P c orrelations relating to the bridging chain are observed. There is also firs t evidence for the third neighbour correlation, P(OP)O, at similar to 2.8 A ngstrom. A residual feature in the neutron diffraction data. present at sim ilar to 1.8 Angstrom, is interpreted as Al-O correlations on the basis of A l-27 MQMAS NMR experiments. This aluminium impurity originates from the use of aluminium oxide crucibles used in the glass synthesis and is shown to e xist as a mixture of octahedral, tetrahedral and penta-coordinated Al-O spe cies. No structural perturbations of the overall network were observed with varying sample composition.