UC San Diego

Defects are a key component in fine tuning the behavior of materials for technological applications. In this thesis, we explore the effect of planar (in the form of surfaces and grain boundaries) and point defects on material properties. We will use density functional theory (DFT) calculations to uncover the physics behind defect modified material properties and develop high-throughput databases. This thesis is broadly divided into three topics.

The first topic (Chapters~\ref{surface_energy_section} and~\ref{work_function_section}) focuses on developing a high-throughput database for surface energy and work function. This study covers elemental crystalline solids of over 100 polymorphs and over 70 elements. Our database is rigorously validated against previous experimental and computational data where available. We propose a weighted work function and surface energy based on the Wulff shape that can be compared to measurements from polycrystalline specimens. We show that the weighted work function can be modeled empirically using simple atomic parameters. Finally, we analyze work function anisotropy with simple bond breaking rules for metallic systems.

The second topic (Chapters~\ref{TaC_section} and~\ref{Mo_GB_section}) is an investigation of the role of metallic dopant segregation in the planar defects of refractory materials with the intention of reducing room temperature brittleness. We use DFT calculations in conjunction with experimental results to demonstrate dopant induced nanocube formaiton in \ch{TaC} which is useful for reducing material porosity during sintering. We also investigate the strengthening/embrittling effects of 29 dopants at the grain boundaries of Mo using DFT and empirical continuum models and assessed several simple atomic parameters as predictors for segregation and embrittlement.

The third topic (Chapters~\ref{V2O3_section} and~\ref{SmNiO3_section}) investigates the role of point defects on the metal insulator transition (MIT) of quantum materials for the purpose of developing energy-efficient neuromorphic devices. Using the PBE+$U$ functional, we explore the effect of intrinsic point defects on temperature and electric field induced MIT in \ch{V2O3}. In corroboration with experimental results, we also investigate the use of H-doping to tune the Ni valency of \ch{SmNiO3} which has broad implications in its MIT properties.