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Title: The Structural Role of Lone PairIons in Novel Glasses
Author: Barney, Emma Ruth
Awarding Body: The University of Warwick
Current Institution: University of Warwick
Date of Award: 2008
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The structures of several glass systems containing the lone pair ions lead or tellurium have been studied using a range of techniques including; neutron diffraction, magic angle spinning nuclear magnetic resonance (MAS NMR), Raman scattering, extended X-ray absorption fine structure (EXAFS) and density measurements. Comparisons with related crystals have also been used to elucidate the role lone pair ions play in glass structure. Studies of lead aluminate glasses (compositions from 64 to 80 mol.% PbO) and the related crystal phase, Pb9AIs0 21 (-69.2 mol.% PbO), using neutron diffraction have shown the crystal to be a good model for glasses with high PbO content. The AI-o peak in the total correlation functions, T(r), indicated that aluminium tetrahedra in the glasses are as well defined as those measured in the crystal. Furthermore, Pb9AIs021 has a coordination number of less than 3 in the region of2.0 - 2.5 A due to a proportion ofthe lead atoms occupying symmetric ionically bonded sites, with higher coordination numbers and longer bonds. Although samples with more than 72.5 mol.% PbO are 3-coordinated in the region, indicating all of the lead atoms are occupying asymmetric lead sites which have short PbO bonds, the coordination number decreases as the PbO content is reduced. There is a corresponding change in the densities of the samples and it is suggested that glasses with similar PbO content to Pb9AIs0 21 contain symmetric lead sites. In refining the crystal structure for Pb9AIs0 21 using neutron diffraction data it was found that the crystal could be formed over a range ofcompositions and a refinement ofthe occupancies gave a refmed composition ofPbs.49±O.Q7Als02o.43±O.13. Within error this composition is chargebalanced. The refined structure gave better agreement with the density ofthe crystal, the 27Al 'MAS NMR, and gave better thermal parameters for the 03 site, which is an unfavourable environment for oxygen atoms, being under-bonded and having a 1800 AI-0-AI bond angle. This site was found to have an occupancy of 0.76. . 10 and 20 mol.% Na20 sodium tellurite glasses were studied using neutron diffraction and, using data for potassium and lithium tellurites of equivalent composition, the bond length, coordination number and disorder ofthe first Na-o coordination shell were extracted. It was shown that the sodium environment ofthe 20 mol.% glass was equivalent to that ofthe stable crystal structure Na2Te409. However, a comparison of T(r) for the glass, the stable crystal and the meta-stable crystal demonstrated that the tellurium environment in the meta-stable crystalline phase ofNa2Te409 was a better model for the glass than the stable crystal. A range of tellurium borate glasses were made, and it was shown that the glasses phase separate, and the compositions measured did not agree with the compositions reported in the literature for glaSses with similar densities and fraction of four-coordinated boron (N4). Comparison ofT(r) measured using neutron diffraction with T(r) for alkali tellurites ofsimilar composition demonstrate that the tellurite network evolves differently with the addition of a network former, with enters the network, than with a modifier, which allows the formation of non-bridging oxygen atoms, reducing the Te-0 bond lengths and breaking up the network . In addition, tellurite network changes from 4- to 3-coordinated more quickly with the addition ofB20 3 than with the addition ofa modifier, and the Te - 0 peak in T(r) is more symmetric.
Supervisor: Not available Sponsor: Not available
Qualification Name: Not available Qualification Level: Doctoral
EThOS ID:  DOI: Not available