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Title: Modelling of inorganic materials : applications to thin films and to radiation damage
Author: Stein, Michael Joseph
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2011
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This thesis looks at two different problems involving halide and oxide systems: The first problem concentrates on the form of CaF 2 films upon a BaF 2 substrate with the objective of determining the structure of CaFzlBaF2 boundaries in CaFzlBaF2 hetero-films. CaF2IBaF2 hetero-film conductivity is much larger than that found for bulk CaF2 and BaF2 systems. Additionally conductivity within these hetero-structures generally increases in proportion to the number ofCaF2IBaF2 boundaries present. The boundary between the CaF2 and Baf2 layers is strained since both bulk BaF2 and CaF2 have different lattice parameters. This lattice mismatch must be overcome at the boundary if a hetero-structure is to be stable. It is assumed that the strain in the CaF2IBaF2 boundary is reduced via the formation of a number of stress relieving defects. Since fluorites conduct via a defect mechanism it is thought that the accumulation of defects at the interface causes the increase in conductivity seen in these hetero-structures. At present little is understood about the form of the boundaries and it is therefore difficult to draw strong conclusions regarding the cause of the increased conductivity in these systems when compared to bulk. By using pair potentials and DFT I find that the structure of the films is dictated by the need to maximise the coordination of ions within the film and to find an optimal interaction with the substrate. I find that large coverages form domains as opposed to forming uniform films. Additionally I see that a number of the F- ions are attracted away from the BaF2 substrate towards the CaF2 layer. These findings indicate that the structure of CaF2IBaF2 boundaries in CaF2IBaF2 hetero-films is not atomistically flat as previously assumed. Additionally the attraction of F: ions towards the Caf'2 layers indicates that BaF2/CaF2 boundaries may form space-charge zones which lend support to the possibility of charge transfer of F' ions from BaF2 layers to CaF2 layers. The second section focuses on the development of a method to study the radiation damage caused by alpha decay in pyrochlore systems via the analysis of molecular dynamics simulation results. The nature of radiation resistance in Gd2B207 pyrochlores varies with the nature of the B cation. Two pyrochlores, Gd2Ti207 and Gd2Zr207, differ remarkably in their resistance. Gd2Ti207 amorphises under irradiation whilst Gd2Zr207 generally retains its pyrochlore structure. Many experimental and theoretical studies have attempted to rationalise the difference in resistance between the two pyrochlores with little success. Our group have performed molecular dynamics simulations to study the effect of alpha decay on both pyrochlore systems. These systems are typically very large and difficult to analyse. Many traditional methods for studying damage in solids, such as radial distribution functions, cannot be applied successfully to pyrochlore systems. I have utilised Steinhardt's bond orientational order parameters to devise a method to study the two pyrochlore systems. The approach I have devised enables in depth study of the pyrochlore structure and can detect the relative quantity of damaged and undamaged material in the two pyrochlore systems. The proportion of damaged and undamaged material in the two pyrochlore systems enables us to draw conclusions regarding the radiation resistance of both materials. My studies lead me to conclude that the main difference in radiation resistance between the titanate and ziroconate systems is due to the nature of the oxygen mobility in both systems.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available