Ab Initio Structure Determination and Property Characterization of High-Pressure Ca-O Compounds and Li2(OH)Br Crystals
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UNLV Theses, Dissertations, Professional Papers, and Capstones 5-1-2016 Ab Initio Structure Determination and Property Characterization of High-Pressure Ca-O Compounds and Li2(OH)Br Crystals Christopher Higgins University of Nevada, Las Vegas Follow this and additional works at: https://digitalscholarship.unlv.edu/thesesdissertations Part of the Physics Commons Repository Citation Higgins, Christopher, "Ab Initio Structure Determination and Property Characterization of High-Pressure Ca-O Compounds and Li2(OH)Br Crystals" (2016). UNLV Theses, Dissertations, Professional Papers, and Capstones. 2680. http://dx.doi.org/10.34917/9112078 This Thesis is protected by copyright and/or related rights. It has been brought to you by Digital Scholarship@UNLV with permission from the rights-holder(s). You are free to use this Thesis in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s) directly, unless additional rights are indicated by a Creative Commons license in the record and/ or on the work itself. This Thesis has been accepted for inclusion in UNLV Theses, Dissertations, Professional Papers, and Capstones by an authorized administrator of Digital Scholarship@UNLV. For more information, please contact [email protected]. AB INITIO STRUCTURE DERTERMINATION AND PROPERTY CHARACTERIZATION OF HIGH-PRESSURE Ca-O COMPOUNDS AND Li2(OH)Br CRYSTALS By Christopher A. Higgins Bachelor of Science – Physics University of Nevada, Las Vegas 2013 A thesis submitted in partial fulfillment of the requirements for the Master of Science – Physics Department of Physics and Astronomy College of Sciences The Graduate College University of Nevada, Las Vegas May 2016 Thesis Approval The Graduate College The University of Nevada, Las Vegas April 12, 2016 This thesis prepared by Christopher A. Higgins entitled Ab Initio Structure Determination And Property Characterization Of High-Pressure Ca-O Compounds and Li2(OH)Br Crystals is approved in partial fulfillment of the requirements for the degree of Master of Science – Physics Department of Physics and Astronomy Changfeng Chen, Ph.D. Kathryn Hausbeck Korgan, Ph.D. Examination Committee Chair Graduate College Interim Dean Michael Pravica, Ph.D. Examination Committee Member Ravhi S. Kumar, Ph.D. Examination Committee Member Pamela Burnley, Ph.D. Graduate College Faculty Representative ii ABSTRACT Ab Initio Structure Determination and Property Characterization of High-Pressure Ca-O Compounds and Li2(OH)Br Crystals By Christopher A. Higgins Dr. Changfeng Chen, Examination Committee Chair Professor of Physics University of Nevada, Las Vegas Theoretical modeling and computational simulations play an important role in materials research. In this thesis, we report on the study of two material systems using various computational methods. The first material system studied here are Cax-Oy compounds under high pressure. Calcium and oxygen are amongst the most abundant elements on Earth, and they form the compound calcium oxide (CaO) that is widely used and very stable at ambient pressures. Although the crystal structure and chemical composition of CaO seems to be simple and well understood, metastable or stable Ca-O compounds with unconventional stoichiometry may exist at high pressures. In this work, first-principles density functional theory calculations and ab initio evolutionary simulations have been used to predict high pressure Ca-O structures. We show that under increasing pressure, the stability of the Ca-O system changes and new materials emerge with different stable or metastable structures. In addition to CaO, we systematically examined structures for Ca, Ca2O, Ca2O3, Ca3O, Ca3O2, Ca7O, CaO2, CaO3, CaO7, and O at high pressures. The high-pressure phase diagram for these compounds is determined along with the electronic density of states and stoichiometry plots. Our energetic analysis identified three new iii stable Ca-O compounds, namely Ca2O3 and CaO2 that are thermodynamically stable above 40 GPa, and CaO3 becomes thermodynamically stable at 150 GPa. The second material system studied in this work is the compound Li2(OH)Br. This compound has been shown by experiment to be a promising solid electrolyte for advanced solid state ionic battery applications. However, the crystal structure of this compound is highly complex and has not been determined accurately. This has impeded its further investigation and potential applications. We modelled the structure effectively using the density functional theory via VASP software. Having successfully determined the correct structure within our model and confirming it by comparison to experimental XRD results, we then performed other calculations to ascertain the characteristics of this compound. The electronic density of states was determined, along with the phonon density of states and the volume vs pressure curve. From the information found within the volume vs pressure plot we then determined the bulk modulus of the material. iv ACKNOWLEDGEMENTS First and foremost I would like to thank Dr. Changfeng Chen for his years of advising and support in my education and research. I feel that through his guidance I have been thoroughly prepared to perform quality research within the scientific community; and I am sure I would not be where I am today without his influence. I would also like to thank Dr. Yi Zhang for his endless patience in helping me to better understand my research. I would also like to acknowledge my research group peers, Andrew Alvarado and William Wolfs. Our hours of discussions and struggles to understand the world of physics have been vital to my education, and I appreciated every minute of it. v TABLE OF CONTENTS ABSTRACT ................................................................................................................................ iii ACKNOLWEDGEMENTS ........................................................................................................ v LIST OF TABLES .................................................................................................................... vii LIST OF FIGURES .................................................................................................................. viii CHAPTER 1: INTRODUCTION ................................................................................................. 1 CHAPTER 2: THEORY ............................................................................................................... 4 CHAPTER 3: STRUCTURAL MODEL AND METHODS ...................................................... 35 CHAPTER 4: RESULTS ............................................................................................................ 37 CHAPTER 5: CONCLUSION ................................................................................................... 62 REFERENCES ........................................................................................................................... 64 CURRICULUM VITAE ............................................................................................................. 69 vi LIST OF TABLES Table 1 …………………………………………………………………………………………40 vii LIST OF FIGURES Figure 1 …………………………………………………………………………………………9 Figure 2 …………………………………………………………………………………………10 Figure 3 …………………………………………………………………………………………11 Figure 4 …………………………………………………………………………………………12 Figure 5 …………………………………………………………………………………………13 Figure 6 …………………………………………………………………………………………23 Figure 8 …………………………………………………………………………………………30 Figure 9 …………………………………………………………………………………………31 Figure 10 ………………………………………………………………………………………..32 Figure 11 ………………………………………………………………………………………..33 Figure 12 ………………………………………………………………………………………..38 Figure 13 ………………………………………………………………………………………..42 Figure 14 ………………………………………………………………………………………..44 Figure 15 ………………………………………………………………………………………..45 Figure 16 ………………………………………………………………………………………..47 Figure 17 ………………………………………………………………………………………..48 viii Figure 18 ………………………………………………………………………………………...49 Figure 19 ………………………………………………………………………………………...51 Figure 20 ………………………………………………………………………………………...53 Figure 21 ………………………………………………………………………………………...56 Figure 22 ………………………………………………………………………………………...58 Figure 23 ………………………………………………………………………………………...59 Figure 24…………………………………………………………………………………………61 ix LIST OF EQUATIONS Equation 1………………………………………………………………………………………..17 Equation 2………………………………………………………………………………………..17 Equation 3………………………………………………………………………………………..24 Equation 4………………………………………………………………………………………..59 x CHAPTER 1 INTRODUCTION Calcium and oxygen are the fifth and first most abundant elements on Earth, respectively. They form the well-known compound calcium oxide (CaO) that has been proven to be a stable compound with very little in the way of polymorphic attributes having only one stable phase at ambient pressures and a second high pressure phase which results in a body centered cubic (BCC) structure observed through experiments [1]. In chapter 2 of this thesis, crystal structures such as BCC are explained in greater detail. Another reason of particular interest is CaO's simple NaCl structure. The Earth's lower mantle can reach pressures up to 140 GPa, resulting in some very interesting high pressure interactions. Recent studies have shown these new interactions to be a keen insight in understanding how the lower mantle may function. Thus, for the purposes of these calculations we have focused on the simulations that run up to 150 GPa. The specific software used