Neutron diraction analysis of the atomic short range order in lead gallate glasses Alex C. Hannon a, * , John M. Parker b , Behnam Vessal c a ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK b Department of Engineering Materials, Sheeld University, Mappin Street, Sheeld S1 3JD, UK c Structural Bioinformatics Inc., 10929 Technology Place, San Diego, CA 92127, USA Abstract Neutron diraction 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 eect 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  A. However, an alternative model with similar numbers of PbO 3 trigonal pyramids and PbO 4 square pyramids cannot be ruled out. Ó 1998 Elsevier Science B.V. All rights reserved. 1. Introduction With the rapidly increasing importance of glass for optoelectronic and ®bre applications, there has been a drive to discover new glass-forming systems and to design glasses with optimum properties for speci®c applications [1]. A lot of interest has been given to gallium-containing glasses, both because the addition of gallium dramatically improves glass stability and because the large atomic mass causes a reduction in phonon frequencies resulting in an improvement in ®bre ampli®er eciency. In 1986 Dumbaugh reported the discovery [1] of several heavy metal oxide glass-forming systems, including the system (PbO) x (Ga 2 O 3 ) 1x (x is the molar fraction of PbO). Table 1 summarises the literature reports of this system's glass-forming range for melt quenching. All reports agree that, as the PbO content is reduced, it becomes increas- ingly dicult to avoid crystallisation. Shelby [2] has studied the physical properties of this system as a function of composition, and shown that both the crystallisation temperature and the infrared cuto wavelength undergo an abrupt change at a `critical' composition, x c , be- tween 75 and 78 mol% PbO. It was suggested by Shelby that `some structural change may occur at 75 mol% PbO'. On the other hand, it was pointed out that the density, molar volume, glass transition temperature and visible cuto frequency are all continuous at this composition, suggesting that the structure varies smoothly with composition. Journal of Non-Crystalline Solids 232±234 (1998) 51±58 * Corresponding author. Tel.: +44 1235 445358; fax: +44 1235 445720; e-mail: a.c.hannon@rutherford.ac.uk. 0022-3093/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 3 7 2 - X