Abstract
The structure of different glasses and crystals was studied by combining neutron and high-energy x-ray diffraction with solid-state 31P nuclear magnetic resonance (NMR), and Raman and electron paramagnetic resonance (EPR) spectroscopy.The structure of the metasilicate composition glasses (A2O)x(MO)0.5−x(SiO2)0.5, with A = Na or K, M = Mg, Zn or Ca and x = 1/4 or x = 1/3, was investigated by
combining neutron and high-energy x-ray diffraction. As compared to the structure of magnesium silicate glasses with the enstatite or diopside composition, or the structure of Mg or Zn aluminosilicate glasses with 50 mol% silica, it is found that the alkali ions promote the formation of four-coordinated Mg2+ and Zn2+ species. Based on the Si atom Q⟨n⟩ speciation, where ⟨n⟩ denotes the mean number of bridging oxygen atoms per tetrahedral SiO4 unit, no compelling evidence could be found for a network-forming role for the four-coordinated Mg2+ and Zn2+ ions.
The structure of glasses that covered a large range of compositions in the (TiO2)x
(Nb2O5)(1−x−y) (P2O5)y system was investigated using neutron and high-energy x-ray diffraction. Due to the complexity of the structure, the structure of two crystalline standards and three binary titanophosphate glasses were studied, providing detailed insights into the local environment of Nb-O, Ti-O and P-O bonds.
The diffraction study of the two crystals, NbOPO4 and NaNbO3, aided in the interpretation of the niobium coordination environment. At the same time, it proved the value of using neutron and x-ray diffraction as insightful techniques which can fully resolve structures.
Three titanium phosphate glasses, (TiO2)x(P2O5)1−x with x = 0.715, 0.741, and
0.750, were studied by combining neutron and high-energy x-ray diffraction with solidstate 31P nuclear magnetic resonance (NMR), and Raman and electron paramagnetic resonance (EPR) spectroscopy. Site-specific information was thereby gained on the composition-dependent local structure of the P, Ti, and Nb atoms. The results were interpreted with the aid of a structural model developed for the binary titanophosphate glasses, gathering information from the diffraction and spectroscopic techniques. This model indicates a completely depolymerized network with all phosphorus atoms forming orthophosphate groups, leading to a network of Ti-centred polyhedral units. The Ti-O coordination number increases from 5.32(7) to 5.49(7) with increasing x, suggesting the coexistence of two-coordinated O(II) and three-coordinated O(III) oxygen
atoms. The contributions to the 31P magic angle spinning NMR spectra from
orthophosphate groups, with or without O(III) atoms, were identified. The population of the O(II) and O(III) atoms was quantified through the development of an analytical model for the glass structure, which also provides the composition dependence of various oxygen-entered structural motifs.
This methodology offers a reference point for investigating the structure of other glassforming materials based on networks of higher-coordinated polyhedral units.
| Date of Award | 19 Feb 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Philip Salmon (Supervisor), Anita Zeidler (Supervisor), Gabriel J. Cuello (Supervisor), Gavin Vaughan (Supervisor), Randall E. Youngman (Supervisor) & Bruce Aitken (Supervisor) |