Understanding Reactions and Processes in Solid-State Materials at the Atomic Level

Abstract

This work uses modern computer simulation methods to investigate the thermodynamics and kinetics of processes and reactions in solid-state materials. First, the thermodynamics of the Ba2+ substitutional defect in MgO and the Ag substitutional defect in Cu metal are studied using classical and density functional theory lattice statics and lattice dynamics within the quasi-harmonic approximation. The computed defect energies show that the temperature variation of the defect energies is significant and not negligible as often assumed. We compare the defects in finite-size clusters with those in the bulk. While defect energies of larger clusters are closer to those in the bulk, the interfaces present in finite-size clusters give rise to differences in the degree of structural relaxation. Secondly, we emphasise the geochemical importance of such defect modelling for explaining the partitioning behaviour of trace elements between minerals and melts and explore the factors which are crucial in controlling partitioning. Lattice statics calculations were used to compute energies for the incorporation of various trace elements in CaO and in diopside. The defect modelling was also used to uncover the serious limitations of simple lattice strain models, e.g., the poor description of lattice strains, the use of the invariant cation radii, the oversimplified assumption of the incorporation mechanisms and the disregard for the role of melt species when describing trace-element partitioning.
Finally, we turn to examine the kinetics of some zeolite-catalysed reactions in the context of macromolecular rate theory. Many zeolite-catalysed reactions exhibit non-Arrhenius behaviour in which the rate of reaction is lower than expected at higher temperatures. Based on similarities with enzyme catalysis, we suggest a negative change in the heat capacity of activation is a possible explanation of the negative curvature of the Arrhenius plots for these zeolitic reaction rates. The classical and ab initio calculations based on quasi-harmonic lattice dynamics, molecular dynamics and metadynamics have been employed to investigate the temperature dependence of the free-energy barriers and rates of several diffusion processes in MgO and zeolitic frameworks. The rates of the Mg2+ vacancy migration in MgO, the diffusion of an ethene molecule through an LTA zeolite pore and the diffusion of ethene molecules in the LTA framework are predicted to show deviations from the classical Arrhenius law, in line with macromolecular rate theory. Since macromolecular rate theory proves useful for understanding enzyme thermoadaptation and designing new enzymes with desirable temperature-dependent properties, we hope our findings will help in the design and synthesis of novel materials for special purposes.

Date of Award	27 Sept 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Neil L Allan (Supervisor)