Takada, Akira; (1994) Computer modelling of crystalline and vitreous boric oxide. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

There is a long standing controversy concerning the structure of vitreous B2O3. Some experiments have suggested that the boroxol ring (6-membered ring) is a dominant structure feature. However, molecular dynamics simulations using the empirically-derived pair-potentials have failed to demonstrate the presence of boroxol ring. This difficulty seems to be due to the complexity of boron-oxygen bonding, which includes not only the partial covalency, but also allows the boron coordination number to change depending on its environment. In the first half of this thesis, it is shown how quantum-mechanical calculations using a range of techniques (including periodic ab-initio Hartree-Fock and LDF) explain the nature of the structure and bonding in boric oxide. Although these calculations each have their limitations, their results are in accordance with accumulated concepts regarding the nature of the structure and bonding in boric oxide. In the latter half of this thesis, we develop a new many-body potential model which can reproduce the structures of crystalline and vitreous boric oxide. As a transferable many-body potential could not reproduce the vitreous structure of B2O3, we developed a potential in which the pair terms were changed depending on the environment, in order to reproduce not only internal coordinates but also the relative stability (i.e. the difference in total energies) of the two phases of the crystalline material. The inclusion of a dependence of the pair-potential terms on the coordination number agrees with classical concepts of chemical bond strength. Parameters for this potential model are derived for crystalline boric oxide and adapted to the vitreous phase. The simulated structural model is checked by comparison with X-ray and neutron scattering data. We find that this final theoretical study not only confirms the existence of the boroxol ring, but also leads to proposals for structural aspects of the intermediate-range order in boric oxide.

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