Studying Bonding And Electronic Structures Of

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Studying Bonding And Electronic Structures Of

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STUDYING BONDING AND ELECTRONIC STRUCTURES OF MATERIALS UNDER EXTREME CONDITIONS
A DISSERTATION SUBMITTED TO THE DEPARTMENT OF APPLIED PHYSICS
AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
Shibing Wang August 2011

© 2011 by Shibing Wang. All Rights Reserved. Re-distributed by Stanford University under license with the author.
This work is licensed under a Creative Commons AttributionNoncommercial 3.0 United States License. http://creativecommons.org/licenses/by-nc/3.0/us/ This dissertation is online at: http://purl.stanford.edu/zr349qb7986
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I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy.
Wendy Mao, Primary Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy.
Ian Fisher, Co-Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy.
Anders Nilsson
Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost Graduate Education
This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives.
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Abstract
Recent advances in high pressure diamond anvil cell techniques and synchrotron radiation characterization methods have enabled investigation of a wide range of materials properties in−situ under extreme conditions. High pressure studies have made significant contribution to our understanding in a number of scientific fields, e.g. condensed matter physics, chemistry, Earth and planetary sciences, and material sciences. Pressure, as a fundamental thermodynamic variable, can induce changes in the electronic and structural configuration of a material, which in turn can dramatically alter its properties. The novel phases and new compounds existing at high pressure have improved our basic understanding of bonding and interactions in condensed matter.
This dissertation focuses on how pressure affects materials’ bonding and electronic structures in two types of systems: hydrogen rich molecular compounds and strongly correlated transition metal oxides. The interaction of boranes and hydrogen was studied using optical microscopy and Raman spectroscopy and their hydrogen storage potential is discussed in the context of practical applications. The pressureinduced behavior of the SiH4 + H2 binary system and the formation of a newly formed compound SiH4(H2)2 were investigated using a combination of optical microscopy, Raman spectroscopy and x-ray diffraction. The experimental work along with DFT calculations on the electronic properties of the compound up to the possible metallization pressure, indicated that there are strong intermolecular interactions between SiH4 and H2 in the condensed phase. By using a newly developed synchrotron x-ray spectroscopy technique, we were able to follow the evolution of the 3d band of a 3d transition metal oxide, Fe2O3 under pressure, which experiences a series of structural, electronic and spin transitions at approximately 50 GPa. Together with theoretical
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calculations we revisited its electronic phase transition mechanism, and found that the electronic transitions are reflected in the pre-edge region.
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Acknowledgement
The completion of this dissertation and my Ph.D. study is indebted to many people who have guided and helped me both in academia and in life. I am immensely grateful to my principal adviser and mentor Professor Wendy Mao, who has every talent to revive my passion to science and make my Ph.D. journey a rewarding and enjoyable experience, and to Ho-kwang Mao whose scientific insights always inspire and enlighten me, and to Agnes Mao who provides her support and encouragement along the way.
I would also like to thank my committee members: Professors Ian Fisher, Ted Geballe, Evan Reed, Bruce Clemens and Anders Nilsson for guiding me through my Ph.D. study, asking profound yet important questions at my defense and helping improve the overall quality of this dissertation.
In addition, Professors Zhi-Xun Shen, Tom Devereaux, Alberto Salleo, Yi Cui, Chi-Chang Kao, James Harris, Zhenan Bao, Mike McGehee and Kelly Gaffney have also given me great advice at various stages of my graduate study, most of which I have seriously taken and implemented.
Many thanks to my collaborators who have taught me enormous knowledge and skills of research: Yang Ding, Jinfu Shu, Tom Autrey, Adam Sorini, Cheng-Chien Chen, Xing-Qiu Chen, Yuming Xiao, Paul Chow, Alexander Goncharov, Nozomu Hiraoka, Hirofumi Ishii, Yong Cai and Chong-Long Fu, and to Extreme Environments Laboratory members: George Amulele, Yu Lin, Maria Baldini, Maaike Kroon, Natasha Filipovitch, Hongwei Ma, Gabriela Farfan, Yingxia Shi, Arianna Gleason, Shigeto Hirai, Wen-Pin Hsieh and Qiaoshi Zeng.
I would also like to mention a few persons whose dedication to youth education
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and enchanting personalities have greatly shaped who I am now and have led to my pursuing a scientific career. They include my high school science and math teachers Ms. Dongyun Li, Ms. Qiuhui Xu and Ms. Dan Gao, my undergraduate physics and math professors Bangfen Zhu, Yunqiang Yu and Shutie Xiao.
This dissertation is dedicated to my parents, who are my role models in work and in life. Their unconditional love, encouragement and moral support are always indispensable to me.
Finally, I am deeply grateful to my husband Diling and my son Juhua. Besides the joyful companion inside and outside of graduate school, Diling’s high standards and sharp critiques help me grow into a better experimentalist, while Juhua with his courage, persistence and sheer curiosity constantly reminds me to stay young and stay foolish. Shibing Wang Menlo Park, August, 2011
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Contents

Abstract

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Acknowledgement

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1 Introduction to High Pressure

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1.1 Achieving high pressure with a diamond anvil cell . . . . . . . . . . . 3

1.2 Pressure measurement . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Experimental methods

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2.1 Optical microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Raman spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.3 X-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.4 X-ray spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3 Boranes and hydrogen

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3.1 Decaborane and hydrogen . . . . . . . . . . . . . . . . . . . . . . . . 19

3.2 Ammonia borane decomposition . . . . . . . . . . . . . . . . . . . . . 27

3.3 Calculation of hydrogen storage capacity . . . . . . . . . . . . . . . . 30

3.4 Energy intensity calculation . . . . . . . . . . . . . . . . . . . . . . . 32

4 Silane and hydrogen

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4.1 Metallization of hydrogen . . . . . . . . . . . . . . . . . . . . . . . . 35

4.2 Phase diagram of SiH4 and H2 at lower pressure . . . . . . . . . . . . 41

4.2.1 Materials and methods . . . . . . . . . . . . . . . . . . . . . . 42

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4.2.2 Results and discussion . . . . . . . . . . . . . . . . . . . . . . 46 4.2.3 Further discussion . . . . . . . . . . . . . . . . . . . . . . . . . 55 4.3 Formation of SiH4(H2)2 - a new compound . . . . . . . . . . . . . . . 56 4.3.1 Calculations on SiH4(H2)2 to metallization pressure . . . . . . 57 4.3.2 Computational and experimental details . . . . . . . . . . . . 59 4.3.3 Results and discussions . . . . . . . . . . . . . . . . . . . . . . 60 4.3.4 Comparison with other calculations . . . . . . . . . . . . . . . 67

5 Transition metal oxides

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5.1 Effects of pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

5.2 High pressure x-ray absorption study of Fe2O3 . . . . . . . . . . . . . 71

5.2.1 Introduction to Fe2O3 . . . . . . . . . . . . . . . . . . . . . . 71

5.2.2 Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

5.2.3 Theoretical interpretation and discussion . . . . . . . . . . . . 76

5.2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

A Correlation functions of hydrogen

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A.1 Properties of solid hydrogen . . . . . . . . . . . . . . . . . . . . . . . 81

A.2 Correlation function and infrared spectra . . . . . . . . . . . . . . . . 82

A.3 Correlation function and Raman spectra . . . . . . . . . . . . . . . . 84

A.4 Comparison between infrared and Raman . . . . . . . . . . . . . . . . 86

Bibliography

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List of Tables
1.1 Gibbs free energy change for different compounds with different external pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
4.1 Lattice parameter and atomic positions of SiH4(H2)2 . . . . . . . . . 60 5.1 Crystal field splitting energy (CFSE) of Fe2O3 as a function of pressure. 75
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