Ainsworth, RI; (2014) Modelling phosphate-based glasses for biomedical applications. Doctoral thesis (PhD), UCL (University College London).

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Abstract

In this thesis, we present the application of theoretical methodologies to model several compositions of phosphate-based glasses (PBGs) for biomedicine. Quantum mechanical calculations of single crystal phosphorus pentoxide, P2O5, have been conducted using plane wave density functional theory. A rigorous structural, mechanical and electronic characterization of the two most stable phases, o’(P2O5)∞ and o(P2O5), showed both to be highly elastically anisotropic due to structural features. Lӧwdin atomic charge and valence charge density analysis shows mixed ionic and covalent character in both phases. A formal charge, shell-model force field, has been parameterized to reproduce the structural and mechanical properties of o’(P2O5)∞. This has been used to conduct classical molecular dynamics simulations of amorphous P2O5-CaO-Na2O systems, via a melt-quench protocol. Dependent on composition, phosphorus atoms are primarily Q1 and Q2. Moreover, calcium ions coordinate to a significantly higher proportion of non-bonded oxygens than sodium. Born-Oppenheimer and classical molecular dynamics simulations of amorphous P2O5-CaO-Na2O-Ag2O systems reveal a distorted octahedral / trigonal bi-pyramidal coordination environment for silver. An increase in the phosphorus to bonded oxygen bond disorder, and disproportionation in the medium-range structure, following the relation 2 Q2 → Q1 + Q3, is evidenced upon Ag-doping. The influence of titanium on the structural, mechanical and electronic properties of PBG has been investigated via Born-Oppenheimer molecular dynamics and theoretical 31P chemical shieldings calculations. Upon Ti-doping, a depolymerization of the phosphate network is offset by the formation of P-O-Ti and Ti-O-Ti linkages. The reconstructed theoretical 31P NMR spectra compare well to experimental spectra, suggesting that the unimodal spectral peak comprises Q1 - 4 phosphorus. The bulk modulus rises from 38.96 GPa in PBG to 43.94 GPa for Ti-PBG, due to a more cross-linked glass network. Density of states calculations show a reduction in the band gap from ~3.3 eV to ~2.1 eV upon Ti-doping.

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