Structure of tellurite glasses-effects of K2O or P2O5 additions studied by 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 tellurite glasses with additions of K2O or P2O5 (max. 16 or 32 mol%). Since the P-O bond lengths do not interfere with the Te-O peaks the Te-O and O-O correlations are approximated by Gaussian fitting of the x-ray and neutron correlation functions up to lengths of 0.28 nm. In the case of K2O-TeO2 glasses reasonable assumptions are made for the K-O first-neighbour peaks. Te-O and O-O coordination numbers are four and five for the glasses of compositions close to pure TeO2 which indicates formation of a three-dimensional network of corner-connected TeO4 trigonal bipyramids. The tellurite network groups are TeO4 and TeO3 units in K2O-TeO2 glasses and TeO4 and TeO5 units in P2O5-TeO2 glasses. Additional Te-O neighbours found at 0.23 nm < r < 0.26 nm for K2O-TeO2 glasses suggest the existence of TeO3+1 units and those found at 0.23 nm < r < 0.29 nm for the P2O5-TeO2 glasses indicate the existence of distorted TeO6 polyhedra. Tellurite networks are flexible to form the appropriate coordination environments for quite different ions or groups of opposite charge such as K+ ions and (PO4/2)(-) tetrahedra where change of the TeOn network units provides the needed number of oxygen neighbours.