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Tuning Functional Properties of Pyrochlore and Related Oxides Via Cation and Anion Substitutions

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Tuning Functional Properties of Pyrochlore and Related Oxides Via Cation and Anion Substitutions AN ABSTRACT OF THE DISSERTATION OF Gabriella Giampaoli for the degree of Doctor of Philosophy in Chemistry presented on March 14, 2018 Title: Tuning Functional Properties of Pyrochlore and Related Oxides via Cation and Anion Substitutions Abstract Approved: Mas A. Subramanian Inorganic structures play an important role in materials due to their versatility and diversity. A complete understanding of the structure of a material is vital to enhance the creation of new materials that fill voids in research. The rational design of these compounds is driven by the exploitation of structure-property relationships, resulting in optimized materials and improved devices. This dissertation navigates this interplay for pyrochlore oxides, investigating the effect of substitution on the structure and characterizing the resulting properties. The fundamentals of inorganic chemistry are presented as an introduction to the research of iridium, ruthenium, bismuth and tin containing pyrochlore materials outlined by this dissertation. The 4d and 5d transition metals have received recent attention as potential topological insulators. This property is driven by the spin-orbit interaction that increases with atomic number. This led to the substitution of various cations in the general pyrochlore formula 2+ 2+ 1+ Bi2-xMxIr2O6+y (M = Ca , Co and Cu ), as well as phases with related structures: Ca2Ru2O7 and Bi2-xCaxRu2-xIrxO6+y. These samples were synthesized via solid state and hydrothermal reactions, resulting in materials with similar structures and properties. In the calcium containing iridium pyrochlore Bi2-xCaxIr2O6+y, the limit of substitution was found to be ~x = 1.0, where the calcium was found to shift from the ideal position to occupy the higher multiplicity 96g site, while pairing with an oxygen vacancy. For higher calcium concentration samples calcium occupied both the 16d and 96g sites. The samples were found to be paramagnetic and metallic. In the related sample Ca2Ru2O7, in the absence of the shared A site, the calcium was found to occupy the ideal site. Cobalt substituted samples were found to have a limit of substitution of ~x = 0.5, and magnetic susceptibility was measured. Copper containing samples (Bi2-xCuxIr2O6+y) were found to have a limit of substitution of ~x = 0.5, and a similar structure to the calcium containing samples. In this preliminary neutron refinement, the copper was found to shift to the 96g position from the ideal 16d, occupying both positions at higher copper content. The oxygen sites were practically fully occupied for both the x = 0.2 and 0.4 samples, suggesting the presence of Cu1+ in the system. Further analysis is required to determine the presence and valency of both iridium and copper in the system. As with the calcium analogue, the samples are paramagnetic and metallic. The Bi2-xCaxRu2-xIrxO6+y system showed a limit of substitution of ~x = 1.0. Both the x = 0.5 and 1.0 samples were paramagnetic. Sulfur and Fluorine substituted Sn2Nb2O7 pyrochlore pigments were synthesized to produce brilliant red/orange compounds as an environmentally friendly alternative to other toxic, inorganic, red/orange pigments. Neutron diffraction structure refinements confirmed that the compositions crystallized in the pyrochlore structure, showing deviations from ideal stoichiometries with a large number of vacancies and severe local structure distortions at the A and O' sites. Chemical analysis confirmed the presence of sulfur and fluorine in the pyrochlore structure and 119Sn Mössbauer analysis confirmed the presence of both Sn2+ and Sn4+ in these compounds. Optical characterization was performed through absorbance, reflectance, and color meter measurements. The compounds were found to have ~60-95% reflectance, having added functionality as “cool” pigments. The estimated band gap was determined to be in the range of 2.35 to 2.05 eV, decreasing with increasing sulfur content, consistent with the color change from yellow to orange/red. Sn2TiNbO6F1-xClx phases were made from x = 0-1.0, resulting in increasing lattice parameter and increasing band gap with chlorine substitution. This solid solution can also potentially find application as a “cool” pigment. The solid solution Sn2Ti1-xCrxWO7-xFx was synthesized from x = 0-1.0, resulting in a decreasing lattice parameter upon chromium addition. The samples were paramagnetic, with magnetic moments from 3.108 μB to 3.513 μB for x = 1 to 0.2. Many materials in the general series Bi2-xCaxM2-xM'xO7 (M = Ti, Hf, Sn; M' = Sb, Nb, Ta) were prepared for the first time and were characterized via their structure and dielectric properties resulting in extremely low loss dielectric materials with dielectric constants from 16-98. The materials were both largely temperature and frequency independent. The lattice parameters of the quaternary pyrochlores in the series BiCaMM'O7 showed an expected linear relationship with the ionic radii of the substituted elements. The solid solution Bi2- xCaxSn2-xSbxO7-y, was analyzed via neutron diffraction, where the limit of the solid solution was close to ~x = 1.3. Until this value, the compounds crystallize in the pyrochlore phase, forming a mixture of the pyrochlore phase and the Ca2Sb2O7 weberite phase beyond this value. The refined formulas for the nominal calcium content x = 0.5, 1.0, and 1.5 in this solid solution were Bi1.36Ca0.64Sn1.60Sb0.40O6.88, Bi0.82Ca1.18Sn1.10Sb0.90O6.86, and Bi0.76Ca1.24Sn1.06Sb0.94O6.85 respectively. ©Copyright by Gabriella Giampaoli March 14, 2018 All Rights Reserved Tuning Functional Properties of Pyrochlore and Related Oxides via Cation and Anion Substitutions by Gabriella Giampaoli A DISSERTATION submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Presented March 14, 2018 Commencement June 2018 Doctor of Philosophy dissertation of Gabriella Giampaoli presented on March 14, 2018. APPROVED: Major Professor, representing Chemistry Chair of the Department of Chemistry Dean of the Graduate School I understand that my dissertation will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my dissertation to any reader upon request. Gabriella Giampaoli, Author ACKNOWLEDGEMENTS I would first like to express my sincere gratitude and appreciation to my research advisor, Dr. Mas Subramanian, who has made it possible for me to complete this monumental task. He has shown incredible guidance and support throughout my tenure at Oregon State University and has brought countless opportunities for learning and growth in this time. From conferences and publications to outreach opportunities, he has gone above and beyond to increase my research and teaching skills and help me succeed in future endeavors, and I would not be the scientist I am today without his mentorship. I am grateful to all members of Dr. Subramanian’s group that I’ve had the pleasure of working with over the years. I would like to specifically thank Dr. Jun Li, who was an enormous and amazing resource; sharing her expertise through crystallographic refinement lessons and answering my general research questions. Thank you to Dr. Elena Medina, Dr. Joshua Flynn, Dr. Rosa Gajczyk, Dr. Whitney Schmidt, Dr. Sandhya Kumari, and Dr. Geneva Laurita for taking the time to answer questions and share your knowledge. I would also like to thank Sarah Synnestvedt, Maxwell Wallace, Elizabeth Solbavarro, Brett Duell, Stephanie Ramos, and Joseph Tang who provided a fun and informative work environment. I would like to thank the members of my graduate committee, Dr. Michael Lerner, Dr. May Nyman, Dr. David Cann, and Dr. William Warnes, for giving their time, and reviewing this work. Thank you to the collaborators that I have had the privilege to work with; Dr. Arthur Ramirez, Dr. Lev Zakharov, Dr. Arthur Sleight, and Dr. Raphael Hermann. I appreciate those who allowed access to their instrumentation for the completion of my graduate work; Dr. Janet Tate, and Dr. Alex Chang. The OSU instructional faculty has been wonderful to work with and learn from. Thank you to Dr. Michael Burand, Margie Haak, Dr. Marita Barth, and Dr. Richard Nafshun for teaching me how to teach, and making graduate school a more dynamic experience than I ever expected. I would also like to thank Luanne Johnson, Sarah Burton, Paula Christie, Greg Jones, Jenna Moser, Todd Stuhr, and Rusty Root for all the support in teaching and fulfilling requirements. I would not have been able to complete this program without the love and support of my family and friends. I thank Susie and Paul Giampaoli, Domenico and Aubin Giampaoli, Angelica Giampaoli, Nina Giampaoli, my nephews Paolo, Vicenzo, Isaiah, Henry, Miles and Marshall, Steve Bandettini, Teresa Cimino, Chris Cunningham, Bill King, Lauren and Kevin Lemieux, Paul Nord, Traci Russo, Tyler Nord, Rosemary Schumacher, Betty Wikeen, Emil and Eleanor Nord, Peter and Carole Giampaoli, and John and Marli Nord for being an amazing family that taught me perseverance and supported me throughout my journey. I would also like to thank Haley Crawford, Lindsey Sequeira, Dr. Breland Oscar, Nels Oscar, Brittany and Marrisa Robertson, and Freddy Dopfel for being an amazing support system and wonderful friends. Most importantly, I thank my best friend and fiancé, Jonathan King. I would not be where I am today without your love, kindness, and support. You make me a better person and I can’t imagine my life without you. I can’t find the words to express how much I appreciate you, and I can’t wait to share the rest of our lives together. Thank you for helping me achieve my dreams. CONTRIBUTION OF AUTHORS Chapter 3: Dr. Mas Subramanian, Dr. Jun Li, Dr. Arthur Ramirez and Dr. Arthur Sleight contributed to the preparation and review of the publication on which this chapter was based; Giampaoli, G.; Li, J.; Ramirez, A. P.; Sleight, A. W.; Subramanian, M. A. Bi2- xCaxIr2O6+y Pyrochlore Phases: Structure and Properties with Varied Oxidation State from 3.9+ to 4.3+.
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