Neutron diffraction analysis of the atomic short range order in lead gallate glasses

Neutron diffraction has been used to investigate the structure of two PbO-Ga(2)O(3) glasses with 67 mol% PbO and 80 mol% PbO, respectively. The Ga-O coordination polyhedron is a tetrahedron irrespective of composition. A simple structural model is able to predict the high PbO limit of the glass-forming range and to provide a structural explanation for the abrupt change in physical properties observed between 75 and 78 mol% PbO. The low PbO limit of the glass-forming range is determined by the constraining effect of the Pb-O coordination shell on the bond and torsion angles of the Ga-O network. The total Pb-O coordination number is 3.6 for 67 mol% PbO and 3.4 for 80 mol% PbO. The most likely model for the Pb-O coordination polyhedron involves a PbO(3) trigonal pyramid with r(PbO) in the range 2.25-2.28 A and about half of the lead atoms having a further oxygen at a distance of about 2.5 Angstrom. However, an alternative model with similar numbers of PbO(3) trigonal pyramids and PbO(4) square pyramids cannot be ruled out. (C) 1998 Elsevier Science B.V. All rights reserved.