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Perovskite and Pyrochlore Tantalum Oxide Nitrides: Synthesis and Characterization

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Perovskite and Pyrochlore Tantalum Oxide Nitrides: Synthesis and Characterization Perovskite and Pyrochlore Tantalum Oxide Nitrides: Synthesis and Characterization Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University by Spencer Hampton Porter, B.S. Graduate Program in Chemistry Ohio State University 2012 Thesis Committee Dr. Patrick Woodward, Advisor Dr. Joshua Goldberger Copyright Spencer Porter 2012 Abstract Oxide nitrides are an emerging class of compounds. Perovskite RETaN2O [RE = La (Imma), Ce (Pnma), Pr (Pnma)] as well as ATaO2N[A = Ca (Pnma), Sr (I 4/mcm), Ba (Pm3¯m)] and pyrochlore RE 2Ta2N2O5 where RE = Ce, Pr (both: Fd3¯m) have been synthesized by solid state and solution-based methods. Crystal structures solved by powder XRD and NPD for all the rare earth analogs (La, Ce, Pr) are reported for the first time. Studies on the preparation techniques of oxide nitrides in both, bulk powder and film format, has shown that solution based precipitation tech- niques decrease crystallite size, increase reactivity, and enable isomorphic films by sedimentation processes. Computational studies on anion ordering generated a library of ordering models herein and finds that, like O2N com- pounds, an ordered cis orientation and out-of-center tantalum displacement provide the most stable model for perovskites with an -N2O anion stoi- chiometry. UV-Vis diffuse reflectance reveals band gaps for CeTaN2O (2.0 eV), PrTaN2O (2.0 eV), Ce2Ta2N2O5 (3.0 eV) and Pr2Ta2N2O5 (3.3 eV). Structure-property relations from calculations elucidate that valence band maximum positions within the compound series is affected by bond lengths and f-orbital contributions. Calculated band gaps decrease as dispersion at the conduction band minimum decreases due to octahedral tilting. These insights shed light on why photocatalytic studies on these perovskite oxide nitrides do not yield appreciable oxygen evolution rates. ii To my family iii Acknowledgements Thank you to all the people along the way who have taken their time to shape and mold me. I am grateful for your efforts and hope this work will show that it was not in vain. To my parents, for always being proud when I share with them my accomplishments, and for being supportive during my failures. They have imbued in me all of the Porter Principles: persistence, patience, and precaution. Life would be much more difficult without these principles. To Dr. Patrick Woodward for his sage advice, fiscal support, and hospitality during my stay in his lab. Thank you. I am grateful for everything he has done for me. Often times research discussions with him translated into real world survival tips, for instance: ...like a good beer, knowledge deserves to be shared with all who can handle it, though too much can make your stomach upset! I appreciate his balance in life and clairvoyance in the field of science (not just chemistry). To Ohio State University Chemistry for awarding me the William Lloyd and Cora Roberts Evans Fellowship and the Department "Excellence in Chemistry" Scholarship. Likewise, I am grateful for the support during my time here as a graduate teaching assistant. Young minds are the catalyst for driving innovation. Dr. Leonard Brillson and Dr. Snezjana Balaz for hard work and countless hours with instrumentation during our collaboration. Dr. Paris Barnes, Dr. Yi-Hsin Liu and Dr. Joshua Goldberger for seeing in me the potential to aid your endeavors. Dr. Zhenguo Huang for insightful discussions on compound solubility and precipitation methods. Teamwork! To Dr. Gordon Renkes, for countless hours of bestowing your XRD tips and tricks upon me. I never knew string, binoculars and a flashlight could do so much. To my dear friends, who provided solace and refuge during down time. For always being down for adventure regardless of where the rabbit hole may lead, some foliage in Columbus may never properly recover. iv To my previous advisors, Dr. Douglas Keszler (Oregon State University) and Dr. Shane Snyder (SNWA/University of Arizona), for encouraging and supporting me in my endeavors then and now. To the Woodward group past and present. Specifically, thank you to Dr. Harry Seibel for taking me under his wing upon first joining the lab. Also, Dr. Graham King, Dr Rebecca Ricciardo, Allyson Fry, Tricia Meyer, and Jennifer Soliz for bestowing copious amounts of Topas Academic magic into my fingertips. To Dr. Matthew Stoltzfus for being a night owl and CASTEP guru. To Ryan Morrow, for being a purebred. Dr. Michael Lufaso for writing SPuDs and TUBERS. Thank you, United States Government, for deciding science is important enough to subsidize tuition in the sciences and for funding grants for research i.e. (NSF DMR - 0907356). Thank you, Oak Ridge National Laboratory Research scientists, Ashfia Huq and Olivier Gourdon, for your help and guidance during my time on campus. Got Neutrons? Thank you Dr. Ram Shehadri for creating furthered learning experiences via ICMR's Preparative Strategies in Solid State Chemistry and Materials Science at UCSB. Thank you American Physics Society and the International Union of Crystallographers for allowing me to share my work to broad audiences in Portland, Oregon, USA and Madrid, Spain. And lastly, to Ohio State University for facilities and research support staff (Larry, Judy, Jennifer, and Francis, Brittany) that enabled my research. I now truly understand what it is like to be part of a world class institution. v Contents List of Figures ix List of Tables xii 1 Introduction 1 1.1 Perovskite Oxide Nitrides . 1 1.2 Other Oxide Nitrides . 2 2 Synthesis and structural characterization of AETaO2N(AE = Ca, Sr, Ba), RETaON2 (RE = La, Ce, Pr), and RE 2Ta2O5N2 (RE = Ce, Pr) 6 2.1 Previous work . 6 2.2 Synthesis . 11 2.2.1 Preparation of Oxide Precursors . 11 2.2.2 Chemicals . 12 2.2.3 Solid State Technique . 13 2.2.4 Solution Based Techniques . 16 2.2.5 Methanol-Chloride Co-precipitation Technique . 19 2.2.6 Acetic-Acid Nitrate-Chloride Co-precipitation Technique . 23 2.2.7 Thermal Ammonolysis . 24 2.2.8 Selective Oxidation . 30 2.3 Octahedral Tilting Symmetry Considerations for Oxide Nitrides . 33 2.4 Structure Determination of Oxide Nitride Perovskites and Pyrochlores . 37 2.4.1 Pyrochlore Ce2Ta2O5N2 and Pr2Ta2O5N2 ............. 40 2.4.1.1 Rietveld Refinements of the X-Ray Diffraction data for Ce2Ta2O5N2 and Pr2Ta2O5N2 .............. 40 2.4.2 Perovskite CeTaN2O and PrTaN2O ................ 45 vi 2.4.2.1 Rietveld Refinements of the XRD data for CeTaN2O and PrTaN2O....................... 48 2.4.2.2 Rietveld Refinements of NPD data for Perovskite RETaN2O where RE = Ce, Pr . 54 2.4.3 Perovskite LaTaN2O......................... 62 2.4.3.1 Rietveld Refinements of NPD data for Perovskite LaTaN2O 62 2.5 Comparison of Bond Distances, Angles, and Tilts in RETaN2O(RE = La, Ce, Pr) and RE2Ta2O5N2 (RE = Ce, Pr) . 73 3 Electronic, Optical, and Photocatalytic Properties in the ATa(O,N)x Compound Class 86 3.1 Electronic Structure For The ATaO2N and RETaN2O Systems (A = Ba, Sr, Ca; RE = Pr, Ce, La) . 86 3.2 Photocatalytic results . 87 3.3 Diffuse Reflectance of RE 2Ta2(O,N)7 and RETa(O,N)3 where RE = Pr &Ce...................................... 88 3.4 A Model System: Lattice Energies for Ordering Types in LaTaN2O... 94 3.4.1 Theory vs. Experiment . 100 3.5 Calculation of Electronic Structure for the ATaO2N and RETaN2O Sys- tems Where (A = Ba, Sr, Ca and RE = Pr) . 106 3.5.1 Conduction and Valence Band Trends . 107 3.5.2 Valence Band Positions . 110 APPENDIX A: KUBELKA-MONK TRANSFORMED UV-VIS PLOTS OF OXIDE NITRIDES PREPARED BY DIFFERENT SYNTHESIS ROUTES 115 APPENDIX B: GEOMETRY OPTIMIZED STRUCTURES FOR A VA- RIETY OF LaTaN2O MODELS GENERATED IN CASTEP 120 APPENDIX C: ELECTRONIC BAND STRUCTURES AND PARTIAL DENSITY OF STATES PLOTS FOR A VARIETY OF LaTaN2O MODELS GENERATED IN CASTEP 123 vii APPENDIX D: CASTEP GENERATED ELECTRONIC AND ATOMIC PARAMETERS FOR ATaO2N AND RETaN2O(A = Ca, Sr, Ba; RE = Pr) 129 References 136 viii List of Figures 1.1 NaTaO3 to RETaN2O............................ 3 2.1 RETa(O,N)x structure types . 8 2.2 RETa(O,N)x Structure Type Relations . 10 2.3 Flow chart of different preparation techniques for ATa(O,N)3 compounds 12 2.4 Typical reaction scheme for co-precipitation . 21 2.5 Ammonia decomposition as a function of a) temperature and b) pressure 27 2.6 Optimal ammonolysis conditions . 29 2.7 Compound colors post ammonolysis . 30 2.8 Equipment setup for selective oxidation . 31 2.9 Compounds post selective oxidation . 32 2.10 Tilting symmetry for simple perovskites . 34 2.11 Octahedral arrangements by anion order . 35 2.12 Tilting symmetry for complex perovskites in the RETaN2O and AETaO2N series (RE = La, Ce, Pr; AE = Ca, Sr, Ba) . 36 2.13 Trans anion ordering symmetries for the Imma LaTaN2O model . 38 2.14 Cis anion ordering symmetries for the Imma LaTaN2O model . 39 2.15 Chekcell calculated peaks for the defect fluorite and pyrochlore structure types versus an experimental Ce2Ta2N2O5 pattern . 41 2.16 Rietveld refinement of Ce2Ta2N2O5 ..................... 42 2.17 Rietveld refinement of Pr2Ta2N2O5 ..................... 43 2.18 XRD super structure of CeTaN2O ..................... 46 2.19 XRD super structure of PrTaN2O ..................... 47 2.20 Rietveld refinement of CeTaN2O ...................... 49 2.21 Rietveld refinement of PrTaN2O ...................... 50 ix 2.22 Waterfall plot comparing reflections in RETaN2O NPDs . 55 2.23 Neutron Powder Diffraction Pattern for CeTaN2O, bank 2 . 57 2.24 Neutron Powder Diffraction Pattern for CeTaN2O, bank 5 . 58 2.25 Neutron Powder Diffraction Pattern for PrTaN2O, bank 2 . 59 2.26 Neutron Powder Diffraction Pattern for PrTaN2O, bank 5 . 60 2.27 Optimization of A-site bonding scheme in CeTaN2O........... 63 2.28 Comparing calculated reflections for various models to the neutron pow- der diffraction pattern for LaTaN2O...................
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