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Chapter 1 Introduction and Background Junshan Li Ruddlesden-Popper type phases in the Ln-Sr-Fe-O (Ln = La, Nd; n = 3) system Synthesis & Characterization Junshan Li Master Thesis in Materials, Energy and Nanotechnology Faculty of Mathematics and Natural Sciences UNIVERSITY OF OSLO August 2013 Preface This thesis is the review of my master work and experiments, as an essential part of a two- year MSc program at the University of Oslo. The work was conducted from September 2011 to August 2013 under the NAFUMA (Nanostructure and Functional Materials) research group at the center for Materials Science and Nanotechnology (SMN), Department of Chemistry. Acknowledgments Pursuing a MSc degree in Norway has proved a definite choice. Considerable support and help have been given to make this research possible by those extraordinary people and I cherish this precious opportunity to express my gratitude to them. First, I would like to show my deepest gratitude to my supervisor prof. Helmer and prof. Anja for their wonderful guidance, patience and support on the experiment as well as comments on the thesis. I really benefited a lot in many aspects. Here, I am also very grateful to these fantastic guys who are in the NAFUMA research group, Chris, Pushpaka, Eirini, Per, Marius, Hanyan Song, Yang Hu and David for teaching and helping me, and your advice was helpful especially when I was confused. Many thanks, of course, I want to give to Sindre, Hanne, Østain, Ammun, Jon magnus and Carla, it was so nice to share office and work with you. Your jokes, stories… made my life in Norway more colorful. I would like to thank all my friends who have made these two years of my life unforgettable. Yet, most of all, I want to thank my parents and sister for their spiritual and financial support, which ensure I can finish my study here. I still could feel their love and care even I was very far away from home. University of Oslo, Norway 22 August 2013 I II Abstract In our modern society, the consumption and demand for energy is increasing rapidly. Fuel cells (FCs) provide renewable energy by electrochemically converting chemical energy into electrical energy and heat without direct combustion as an intermediate step. Solid oxide fuel cells (SOFCs) have some advantages over other types of fuel cells. Here, the cathode materials have a large impact on the performance of SOFCs. The Ruddlesden-Popper (RP) types oxides, such as the RP3 phase Ln(Sr,Ca)3(Fe,Co)3O10, are promising cathode materials due to their good mixed ionic and electronic conductivity (MIEC). The objective of this master work has been to synthesize RP3 phase LnxSr4xFe3O10δ (Ln = La, Nd; 0 < x < 4.0) via citric acid method. In addition, the investigation of the crystal structure, thermal behavior and magnetic property of this RP3 type compounds represents a main task. A large number of compositions were attempted synthesized. The crystal structure of the RP3 products consists of a triple perovskite slabs separated by rock salt layer. A novel finding is that certain compositions with 0 < x ≤ 1.0 are phase-pure, hence representing a heterovalent substitution that simultaneously changes the average oxidation state for the iron atoms. The unit cell parameters for the as-synthesized RP3 type compounds are consistent with a tetragonal, space group I4/mmm and with dimensions a = b ≈ 3.85 Å, c ≈ 28.00 Å. The main focus has been on samples with x = 1.0. Among the obtained phase-pure compounds, LnSr3Fe3O10δ (Ln = La, Nd) was selected for preparing derivate materials through tuning the concentration of oxygen vacancies (0 < δ < ~1.5), applying methods of full oxidation at low temperatures, quenching and an oxygen-getter method. Conventional powder X-ray (XRD) as well as Synchrotron radiation X-ray diffraction (SRXRD) data were collected and used as input to Rietveld refinement in order to describe the atomic arrangement of these oxygen deficient samples. The oxygen vacancies are located to the equatorial layer of the central octahedra in the triple perovskite block. The length of the c-axis varies linearly with oxygen vacancy concentration, increasing in length upon decreasing the oxygen content. The Fe-O and La/Sr-O bond length and their bond valence sum (BVS) calculated from the Rietveld refinement are evaluated and discussed to present changes connected with oxygen deficiencies. III Thermogravimetric analysis (TGA) was carried out from 30 oC to 1200 oC with a rate of 10 oC/min in air. For the oxidized samples, oxygen is lost upon heating. However, reintercalation of oxygen occurs reversibly as a function of temperature. The oxygen deficient samples are found to be kinetically stable until some 200 oC, thereafter followed by a dramatic oxygen intercalation until 400 oC. Then a mass loss starts and continues up to elevated temperature of 1200 oC. Reversible weight gain is observed upon cooling due to oxygen intercalation. The magnetization as a function of temperature (M(T)) and field (M(H)) was measured for these two series compounds LnSr3Fe3O10δ (Ln = La, Nd). With different Fe state, ferromagnetic and antiferromagnetic exchange interactions are present in LnSr3Fe3O10-δ and hysteresis loop shows that this series compounds are of paramagnetic at high temperatures , and probably quite generally antiferro- or ferrimagnetic at 5K. IV List of abbreviations RP Ruddlesden-Popper phase RPn Ruddlesden Popper phase with n perovskite layers alternating AO layer FCs Fuel Cells SOFCs Solid Oxide Fuel Cells MLCCs Multi-Layer Ceramic Capacitors MIEC Mixed Ionic and Electronic Conductor/Conductivity HTS High Temperature Superconductor LTS Low Temperature Superconductor RT Room Temperature TGA Thermogravimetric Analysis XRD X-ray Diffraction SRXRD Synchrotron Radiation X-ray Diffraction D Dimension/Dimensional HS High Spin LS Low Spin JT John-Teller CD Charge Disproportionation CO Charge Ordering CN Coordination number NPD Neutron Powder Diffraction PM Paramagnetic FM Ferromagnetic AFM Anti-Ferromagnetic SG Spin Glass ap The dimension of a primitive perovskite cubic cell V PPMS Physical Properties Measurement System MPMS Magnetic Properties Measurement System SNBL Swiss-Norwegian Beam Line ESRF European Synchrotron Radiation Facility TOPAS TOtal Pattern Analysis Solutions GSAS General Structure Analysis System FC Field Cooling ZFC Zero Field Cooling M(T) Magnetization as a function of temperature M(H) Magnetization as a function of external applied field χ Magnetic susceptibility χ-1 Inverse magnetic susceptibility ueff effective paramagnetic moment B.M. Bohr magneton A/B/X Atomic site/cation in Ruddlesden-Popper and perovskite Ln Rare earth element δ Overall oxygen nonstoichiometry per formula unit 10δ Overall oxygen content per formula unit ICSD Inorganic Crystal Structure Database SQUID Superconducting Quantum Interface Device VI Contents Preface ......................................................................................................................................... I Abstract .................................................................................................................................... III List of abbreviations .................................................................................................................. V Contents ................................................................................................................................... VII Chapter 1 .................................................................................................................................... 1 Introduction and background ..................................................................................................... 1 1.1 Introduction to perovskite type compounds ..................................................................... 1 1.2 Perovskite related structures ............................................................................................. 5 1.2.1 Simple related perovskites ........................................................................................ 5 1.2.2 Layered perovskite related compounds ..................................................................... 6 1.3 Defective perovskites ....................................................................................................... 9 1.3.1 Background of defects ............................................................................................. 10 1.3.2 Thermodynamics of defects .................................................................................... 11 1.3.3 Defect situations in perovskite type compounds ..................................................... 12 1.4 Possible applications ...................................................................................................... 13 1.5 Literature review ............................................................................................................ 15 1.5.1 The structure of LnSr3Fe3O10δ ............................................................................... 15 1.5.2 The thermal investigation ........................................................................................ 19 1.5.3 The topotactics involving in the NdSr3Fe3O10δ (0 < δ ≤ 1.5) ............................... 21 1.5.4 Electrical properties ................................................................................................. 23 1.5.5 Magnetic properties ................................................................................................. 24 1.5.6 Other properties ......................................................................................................
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