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Point Defects in Strontium and Barium Titanate from First Principles: Properties and Thermodynamics

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Point Defects in Strontium and Barium Titanate from First Principles: Properties and Thermodynamics ABSTRACT BAKER, JONATHON NEAL. Point Defects in Strontium and Barium Titanate from First Principles: Properties and Thermodynamics. (Under the direction of Douglas L. Irving.) Density functional theory (DFT) calculations and point defect thermodynamics have been used together to investigate the strontium titanate (STO) and barium titanate (BTO) materials systems. While it had long been suspected that iron was the cause of brown coloration in Fe-doped SrTiO3, our calculations prove this assignment and resolve a discrep- ancy between the EPR and defect chemistry communities. The neutral iron substitutional is shown to exhibit two absorption processes, one associated with the conduction band and the other with the valence band, which explain the experimentally observed absorption spectrum. The interaction between the iron substitutional and an iron-oxygen vacancy complex were shown to be critical in correctly reproducing the oxygen pressure depen- dence of the coloration onset. Following this, we examined how and why the metal vacancy behavior differed in strontium and barium titanate. Oftentimes, point defects models are transferred directly between strontium titanate and barium titanate. At the same time, and in apparent disagreement with this, while the A-site vacancy is assumed to dominate in STO, there has been some disagreement about which type of metal vacancy dominates in BTO. We use DFT calculations to elucidate differences in the bond energies for the different sites in each material, which lead to the A-site dominating in STO, while combinations of the A-site, B-site, and B-site-oxygen vacancy complex dominate in BTO, depending on the conditions. We then examined the response of the vacancies to different processing conditions when doped with niobium and iron, two common dopants in these materials. Lastly, we investigate niobium doped STO as a test case for a framework for calculating hydrogen solubility on the basis of point defect energies, doping, and environmental con- ditions. Hydrogen incorporation is little-studied in these materials, and it is paramount to understand whether it plays a role as an ionically conductive species; this work is the first step along that road. © Copyright 2018 by Jonathon Neal Baker All Rights Reserved Point Defects in Strontium and Barium Titanate from First Principles: Properties and Thermodynamics by Jonathon Neal Baker A dissertation submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Materials Science & Engineering Raleigh, North Carolina 2018 APPROVED BY: Elizabeth Dickey Ramòn Collazo Carol Hall Douglas L. Irving Chair of Advisory Committee DEDICATION To my wife Jennifer, for always loving me and encouraging me to do what I’m passionate about. To my parents, grandparents, aunts, and uncles, who have always fostered my passion for science. For my high school NJROTC instructor, LCDR Anthony Negron, USN (ret.) for teaching me self-discipline and showing me that I’m capable of anything if I try hard enough. To Mr. Smith and Mrs. Grooms, for teaching me introductory chemistry and calculus, and starting my academic journey. To Mrs. Ligon, for teaching me to write. To the Laders, for exposing me to foreign languages and cultures. To Dr. Kornev, Dr. Mefford, Dr. Kennedy, and Dr. Lickfield: thank you for mentoring me during my time at Clemson University, and teaching me the basics of scientific research. To my sister Jackie: thank you for putting up with me. It can’t have been easy. ii BIOGRAPHY The author grew up in rural South Carolina with his sister Jackie, in northern Aiken county, and attended high school in the city of Aiken, where he graduated in 2009. His family has always encouraged his love of reading, science, programming, and enjoyment of the outdoors. During those formative years, he enrolled in NJROTC, where he made many lifelong friends and learned self-discipline, and took as many classes on science and math as his high school offered, while beginning to develop basic programming skills and a beginner’s knowledge of electronics. He also participated in an outreach event sponsored by the Clemson University Department of Materials Science and Engineering, where he was able to participate at a very basic level in materials science research as an intern, hosted by Dr. Konstantin Kornev. This enkindled his passion for materials science, as he saw it as the fascinating intersection of many different disciplines. He was also exposed to foreign cultures during this time, participating in a 3 week exchange trip to Germany, where he was hosted by a Turkish family who owned a döner shop. Although he hasn’t had much time for it recently, he enjoys taking long walks in the woods, bicycling, kayaking, photography, and target shooting. He is also an avid reader of science fiction. After graduating from high school, he attended Clemson University for the next four years, where he obtained his Bachelor’s degree in materials science, with a minor in chem- istry. During that time, he worked as an undergraduate researcher for Dr. Olin Mefford IV, developing computer programs to simulate the heating rate of colloidal suspensions of magnetic nanoparticles. During the summers, he worked as an intern at Savannah River National Labs, getting exciting hands-on experience with corrosion research, scanning electron microscopy, vacuum lines, and PVD processes. While at Clemson, he met his wife, Jennifer. Upon graduating, they moved to Raleigh together, where Jonathon began graduate school at NCSU, working under the direction of Dr. Douglas Irving. During his first year there, he greatly expanded his knowledge of computers and programming while learning the basics of using density functional theory software to perform quantum mechanics simulations on solids, and developing a data storage and analysis solution to deal with the groups’ ever increasing amount of first principles data. Later, he and former undergraduate Brian Behrhorst re-engineered one of the group’s core tools, a point defect concentration solver for semiconductors, speeding it up by several orders of magnitude and making it iii vastly more maintainable. Several other groups members have since built on this more maintainable framework to make substantial contributions of their own to this important tool. The author’s research focuses on point defects in barium and strontium titanate, and how it influences the optical and electronic properties of the material. This research forms the body of this dissertation. iv ACKNOWLEDGEMENTS The work presented in this thesis would not have been possible without help from numerous collaborators and friends, the support of friends and family, and generous funding and computing time provided by the Air Force Office of Scientific Research. I would like to thank Professor Douglas Irving for all of his advice and guidance during my graduate career; not only have I grown as a scientist during my time here, I have also grown as a person, and I feel that I have learned many life lessons that will serve me well the rest of my career. I would like to thank my colleagues Preston Bowes, Joshua Harris, Yifeng Wu, Kelsey Mirrielees, Dan Long, Nikki Creange, Brian Behrhorst, Ben Gaddy, and Changning Niu for always being willing to discuss research issues and making themselves available to bounce ideas off of. I would like to acknowledge contributions by Preston Bowes, Joshua Harris, Brian Behrhorst, and Ben Gaddy to software that I now use almost daily. I would also like to thank people who performed experiments complementing my simulations: Dan Long, Nikki Creange, Ali Moballegh, and Biya Cai. I would like to especially thank Professors Ramon´ Collazo and Elizabeth Dickey for always providing guidance and support during this endeavour. Preston Bowes deserves a special additional acknowledgement; in an attempt to split the workload during this project, I ran some of the underlying DFT calculations, and Preston ran others. This project is very much a joint effort between me and Preston. My research was directly funded with generous support from Air Force Office of Scien- tific Research Basic Research Initiatives FA9550-14-1-0264 and FA9550-17-1-0318, through Dr. Ali Sayir’s Aerospace Materials for Extreme Environments program. These grants also provided most of the hefty amounts of computing time required to perform this work. The computers themselves also merit an acknowledgement: The actual DFT calculations were performed on either the North Carolina State University High Performance Com- puter, or the Shepard, Armstrong, or Conrad supercomputers, administered through the Navy Department of Defense Supercomputing Resource Center and the Department of Defense High Performance Computing Modernization Program. During the initial phase of the project, we used the Labrea file server to store and process our results. The Labrea server was eventually replaced with a pair of redundant servers running newer hardware with more storage and more powerful, easy to maintain software: the R2D2 and C3PO file servers, which have safely and securely stored all of our data and performed most of our thermodynamics simulations since early 2017. v TABLE OF CONTENTS LIST OF TABLES .................................................. viii LIST OF FIGURES ................................................. ix Chapter 1 Introduction ........................................... 1 Chapter 2 Literature Review: A Historical Perspective ...................
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