Mellan, TA; (2016) DFT simulations of selected strongly correlated functional materials: electronic and redox properties of vanadium and manganese oxides. Doctoral thesis , UCL (University College London).

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Abstract

A theoretical investigation is presented on three correlated transition metal oxide systems - one oxide of vanadium, a manganate and a manganite. Beyond theoretical interest in the strongly interacting electronic structures, each system is relevant to materials design and developing new technologies. For each material in the thesis, vanadium dioxide (VO2), manganese dioxide (β-MnO2) and lanthanum manganite (LaMnO3), reports are presented on the electronic, structural and thermodynamic properties calculated using density functional theory (DFT). Strongly interacting electrons are a major challenge to modelling techniques, so to begin we examine the capacity of DFT to describe VO2. Analysis is presented on the effect of on-site direct and exchange Coulomb corrections, and the exchange mixing parameter in hybrid functional calculations. VO2 phase transition potential energy surfaces are presented in terms of electronic and structural transition parameters. To access transition thermodynamics, zero temperature imaginary phonon modes are transformed to their transition temperature energies. The phonon and electron contributions to the total transition entropy are calculated, which allows us to discuss the nature of the VO2 phase transition. Surface thermodynamics are presented for VO2. Results predict the energetically favoured surfaces, particle morphologies, and surface terminations as a function of temperature and pressure. The Li and O adsorption thermodynamics of β-MnO2 surfaces are explored from the perspective of reducing cathode over-potentials in the Li-air cell. The surface redox chemistry and electronic structure are studied with and without Li adatoms, leading to predictions on the operation of β-MnO2 as a battery cathode material. For LaMnO3 it is found that intra-orbital exchange corrections to DFT reproduce electronic, magnetic and structural observables simultaneously. The importance of Hund's coupling to the LaMnO3 ground state is explored in detail.

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