A Structural Investigation of Chlorine-Containing and Fluorine-Containing Oxide Glasses Using Molecular Dynamics, Neutron Diffraction, and X-ray Absorption Spectroscopy

Scientific developments have enabled glasses to fulfil an array of applications, from windows to bioactive glasses in regenerative medicine. To further exploit the capability of this versatile material, it is imperative that their structure is understood. In this thesis, the structure of three glass systems containing halides as anions were investigated. The first of these was the intermediate glass former ZnCl2 which was modelled computationally using classical molecular dynamics (MD). The addition of the adiabatic core-shell model was able to account for anion polarisability. This enabled the first fully tetrahedral model of ZnCl2 glass to be attained. While 86% of the ZnCl4 tetrahedral units were corner-sharing, 14% were found to be edge-sharing. The calculated total neutron and x-ray structure factors closely replicated those obtained experimentally in other works. The intermediate glass former ZnCl2 was later compared to the strong glass former SiO2. The main contribution in the first sharp diffraction peaks came from the cation-anion contribution, rather than the cation-cation contribution as previously reported.

Next to be investigated was a CaO-SiO2-CaCl2 glass series. This was to help elucidate the structure of more complex CaO-SiO2-P2O5-CaCl2 chlorine-containing bioactive glass compositions. A glass series was synthesised by collaborators, and compositional analysis in this work revealed that chlorine losses via chlorine volatilisation occurred as HCl. The glass series was studied experimentally using neutron diffraction (ND) and x-ray absorption spectroscopy (XAS) at the Ca and Cl K-edge. By probing the calcium environment using ND and XAS, generally good agreement between the Ca-O and Ca-Cl coordination numbers was achieved. The total correlation functions from neutron diffraction did not exhibit a noticeable contribution around 2.1Å which would have been expected for Si-Cl bonding. Computational modelling was performed using MD with the addition of the adiabatic core-shell model. No Si-Cl bonding was observed, and the calculated total neutron structure factors closely resembled those obtained experimentally. The glass models were found to become phase separated with increasing CaCl2 content to form a biphasic system of calcium silicate and calcium chloride phases. Interestingly, there was a tendency towards phase separation even in glass models containing small amounts of CaCl2.

The remaining glass system, CaO-SiO2-CaF2, was studied to help elucidate the structure of more complex CaO-SiO2-P2O5-CaF2 fluorine-containing bioactive glasses. Following the synthesis of the CaO-SiO2-CaF2 glass series, compositional analysis revealed that fluorine losses due to fluorine volatilisation occurred as HF. The calcium environment of the glasses was probed using ND and XAS at the Ca K-edge. Distinguishing the overlapping Ca-F and Ca-O paths around 2.3Å and 2.4Å respectively was challenging. The glass series was modelled computationally using MD with the addition of the adiabatic core-shell model. The calculated total neutron structure factors closely replicated those from experiment. The glass models also revealed that while fluorine ions overwhelmingly bond with calcium ions, small amounts of Si-F bonding are observed which conceivably cannot be resolved experimentally.