Discovering and Understanding High Performance Materials using Density Functional Theory: Quantum Mechanical Simulations and the Consequences of Symmetry

Abstract

There are two primary ways that atomic level modeling data is used: materials prediction and understanding materials properties. This dissertation work encom- passes two studies, each of which explore one application. Both studies rely on the highly successful density functional theory (DFT) formalism but differ in that two different implementations of DFT are used on two different high performance materials. The first study on bulk magnesium (Mg) metal alloys explores materials prediction and relies on VASP, a commercially maintained plane-wave DFT code which has been used extensively to successfully study a wide range of materials. [1] The approach used in this first study is to ‘experiment’ within computational quantum mechanical simulations to improve the elastic properties of bulk Mg by altering its HCP lattice structure. We systematically study the influence of adding lithium (Li) as an alloy for two reasons: to maintain the lightweight benefits of Mg, and Li naturally occurs in a body centered cubic (BCC) crystal structure. The hypothesis is that an alloy with a more symmetric crystal structure will show im- proved properties, however we do not place any symmetry restrictions on the results of the structure search. We find that the addition of Li to Mg does improve the elastic properties of the resulting alloys; however it does not necessarily increase the symmetry. Five structures are found which belong to the convex hull, three of which are previously unreported. The second DFT study seeks to understand the electronic environment within lead sulfur (PbS) semiconductor nano-structures and utilizes the open-source Octopus code, designed for electron-ion dynamics in finite systems using time-dependent DFT in real time and real space and which has also been bench-marked extensively [2]. The aim of the second study is to understand at the most fundamental levels the impact reduced symmetry has on the electronic states and transitions at the level of the individual IR-light-absorbing quantum dot. We employ three toy models to isolate the impacts of reduced coordination, Pb-rich structures, and Peierls distortions. An in-depth analysis of the bonding through the charge density and electron localization function shows that the metavalent bonding observed in bulk PbS persists in the nanoscale regime. Changing the stoichiometry too far away from Pb:S = 1:1 results in the loss of semiconducting character and an overall metallic character prevails. When we place particular attention on the effects of atomic coordination, we observe enhanced electron localization clustered around the lowest coordinated atoms. Peierls distortions intensify the clustering behavior which lowers the energy of the occupied electronic states and increases the energy of the unoccupied states as deduced from density of states plots. The change in the electron localization is substantial only for a significant amount of low-coordinated atoms. A conclusion is made with an outlook to future work

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