Structure of zinc and niobium tellurite glasses by neutron and x-ray diffraction

Neutron and x-ray diffraction experiments of high resolving power with neutrons from a spallation source and high-energy photons from a synchrotron have been performed on compositional series of binary Zn, Nb and on mixed Zn/Nb tellurite glasses. The Te-O, Zn-O and Nb-O coordination numbers are determined by Gaussian fitting of the first-neighbour peaks in the neutron and x-ray data simultaneously. The transition of TeO₄ to TeO₃ units with increasing fraction of a second component is indicated by decreasing total Te-O coordination numbers. This transition appears different for glasses with ZnO or Nb₂O₅ additions. Details of the Te-O peaks suggest there are two species of Te-O bonds with lengths of ∼0.19 and ∼0.21 nm. The change of their fractions shows excellent agreement with the existence of TeO₄ trigonal bipyramids and TeO₃ trigonal pyramids. All oxygen atoms from ZnO and Nb₂O₅ are used for rupture of Te-O-Te bridges, which is accompanied with a change of nearly all participating TeO₄ to TeO₃ groups. The tendency for a TeO₄ → TeO₃ change decreases for glasses of higher second component content which is accompanied by the occurrence of TeO₄ groups with non-bridging oxygens. The Nb tellurite glasses show transition to network-forming behaviour with the formation of Nb-O-Nb bridges. The fractions of TeO₃ units of ternary Zn/Nb tellurite glasses agree with an additivity behaviour of the modifying effects of ZnO and Nb₂O₅ additions.