The structure of tin borate based glasses

A series of tin borate glass samples was prepared by melting in alumina crucibles, and these were studied by means of. neutron diffraction, Al-27 and B-11 MAS (magic angle spinning) NMR, density, XRF and EDX measurements, The results show that the samples contain about 6.5 mol% Al2O3 from the crucibles, and when this was taken into account the B-O coordination numbers from neutron diffraction showed reasonable agreement with B-11 MAS NMR results. These results serve as a case study for showing the importance of sample composition for a correct analysis of diffraction results for glasses. Die presence of Al2O3 has only a small effect on the number of 4-coordinated borons because few of the aluminium are on 4-coordinated sites. For low SnO content, both the fraction of 4-coordinated borons and the mean B-O bond length increase as SnO is added. However, for greater than 50 mol% SnO the fraction of 4-coordinated borons decreases, but the mean B-O bond length remains constant, and this behaviour of the bond length is as yet unexplained. For the Sn-O coordination, the results may be interpreted as showing that for high SnO content the Sn2+ ions are predominantly on asymmetric, trigonal pyramid SnO3 sites, but that, for low SnO content the Sn2+ ions are on a mixture of asymmetric SnO3 sites and synnnetric, high coordination sites