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Molecular Fullerides Molecular Fullerides by Wilfred Kelsham Fullagar A thesis submitted for the degree of Doctorate of Philosophy of The Australian National University April, 1997 Declaration The research described in this dissertation was undertaken primarily at the Australian National University, under the supervision of Professor John White. Data collection described herein was in three instances performed elsewhere: X-ray synchrotron diffraction data was collected at the Australian National Beamline Facility in Tsukuba, Japan; neutron diffraction patterns were recorded at the Intense Pulsed Neutron Source at the Argonne National Laboratories in Illinois, USA; and inelastic neutron scattering measurements were made at the ISIS Pulsed Neutron Source of the Rutherford Appleton Laboratory, Chilton, UK. Except where otherwise stated, sample preparation and analysis was in all instances performed by the candidate. Note, however, the particularly valuable contributions of Dr. Philip Reynolds and Dr. Lucjan Dubicki in § 4 and § 5, respectively (see Acknowledgments). Funding restrictions prevented the candidate from attending one of the three data collection trips to the Australian National Beamline Facility in Japan, and the relevant diffraction data were measured on his behalf by Dr. Philippe Espeau and Dr. John Watson. Similar restrictions led to the Na2C60, Rb4C60 and Rb3C60(ND3)2.5 TFXA data in § 6 being collected on the candidate's behalf by Dr. Philip Reynolds and Dr. Tony Brown, this being the second of two expeditions to the Rutherford Appleton Laboratory, UK. The associated sample preparations and subsequent data interpretation were by the candidate. None of the work presented in this thesis has been submitted to any other institution for any degree. Wilfred K. Fullagar 28/4/1997 Abstract The closed shell structures of certain all-carbon fragments originally observed in mass spectroscopy experiments leads to the enhanced stability of these species, known as fullerenes, which have excited sufficient interest amongst chemists and physicists over the last decade to warrant the award of the 1996 Nobel Prize for Chemistry to their discoverers. Studies of the stability, symmetry, and consequent remarkable properties of fullerenes began in earnest in 1991 with the development of a technique enabling the production and purification of macroscopic quantities of material. The best known and most widely studied fullerene is the truncated icosahedral C60 molecule, which forms the basis of the present work. One important property of C60 is that it forms salts with sufficiently electropositive species, such as the alkali metals. The resulting salts contain C60 anions and are known as fullerides. Certain of these salts display metallic behaviour, and some superconduct at temperatures as high as 33 K. Three aspects of fulleride research are addressed in this work. These are: i) the preparation, crystal structure determination and superconductivity characterization of several new fullerides, particularly those including ammonia as an additional intercalant, n- ii) the electronic structure of the C60 (n = 1 - 6) anions, as probed by solution-phase near infrared absorption spectroscopy, and iii) the molecular dynamics of a number of fullerides, superconducting and non- superconducting, by inelastic neutron scattering. This work has grown out of an Honours project also concerning C60, the combined duration of the two studies covering essentially the entire history of this widely and competitively studied field. Acknowledgments I owe a great deal to my supervisors, Professor John White, Dr. Graham Heath and Dr. Richard Bramley, for their encouragement and patient enthusiasm throughout my time at the Research School of Chemistry. Professor White is owed particular personal thanks for accepting me as a PhD student in perhaps rather trying circumstances! The contributions of Dr. Philip Reynolds to this work, and to four of the five publications that have arisen from it are nothing short of vital. He introduced me to crystallographic structure refinement techniques (§ 4), and certain of the refinements described in this work were initiated and indeed completed by him. His understanding of neutron scattering has also been extremely valuable. Patience and clarity as a teacher, a deep understanding of general physical chemistry, and an approachable and freindly manner are qualities of his that have been appreciated by all members of the White group, and his input to the latter cannot be understated. Dr. Lucjan Dubicki is another key contributor, and was kind enough to embrace the very challenging problem of the electronic structure in the near-infrared absorption spectra of the C60 anions (§ 5), with the associated wealth of chemical literature, all at extremely short notice. The present understanding of the near infrared absorption spectra as arising from a combination of interelectron repulsion and Jahn-Teller effects is in large measure his achievement, and it is regretted that he was not approached on the topic much sooner. The technical staff at the Research School of Chemistry played a crucial role in this work for their construction and assistance in the design of many and varied pieces of apparatus, ranging from hermetically sealable cryogenic sample cans to the liquid ammonia titrator described in § 3.1.3; Chris Tomkins' input in the design and construction of the latter having been particularly noteworthy. Particular thanks are due to Mr Gordon Lockhart, whose assistance and expertise in the laboratory on innumerable occasions proved him quite indispensable. I would especially like to thank him for teaching me the rudiments of glassblowing, a skill which enabled a high degree of independence in the construction of laboratory apparatus. I thank the Australian Nuclear Science and Technology Organization (ANSTO) for making possible access to the international facilities described in § 8, as well as providing the necessary funding. Finally, this work would have been quite impossible had it not been for the constancy of parents, siblings and friends, whose apparently endless compassion and forbearance in the face of all manner of grievances have always been and continue to be a source of particular strength. Table of Contents 1. SCOPE OF THESIS ..............................................................................................................................................1-1 1.1 REFERENCES..........................................................................................................................................................1-4 2. STRUCTURES AND SUPERCONDUCTING PROPERTIES .......................................................................2-1 2.1 BASIC STRUCTURAL MOTIFS.............................................................................................................................2-1 2.1.1 Face-Centred Cubic Structures ..............................................................................................................2-1 2.1.2 Body-Centred Structures..........................................................................................................................2-6 2.1.3 Other Packing Schemes............................................................................................................................2-8 2.1.4 C60 Molecular Structure Changes ..........................................................................................................2-9 2.1.5 Oligomeric phases .....................................................................................................................................2-9 2.2 SOLID-STATE BAND STRUCTURES.................................................................................................................2-11 2.3 FULLERIDE SUPERCONDUCTIVITY REVIEW..................................................................................................2-18 2.3.1 Relationship between structure and superconductivity...................................................................2-19 2.3.2 Isotope effects............................................................................................................................................2-21 2.3.3 Superconducting Parameters of the Fullerides .................................................................................2-22 2.3.4 Electronic Pairing Model of Fulleride Superconductivity..............................................................2-22 2.3.5 Electron-Phonon Coupling Model of Fulleride Superconductivity..............................................2-24 2.4 REFERENCES........................................................................................................................................................2-28 3. SYNTHESIS AND SUPERCONDUCTIVITY...................................................................................................3-1 3.1 PREPARATIVE METHODS....................................................................................................................................3-1 3.1.1 Vapour-phase intercalation ....................................................................................................................3-1 3.1.2 Preparation via decomposition of alkali-containing compounds...................................................3-3 3.1.3 Preparation by titration...........................................................................................................................3-4 3.1.4 Bomb synthesis .........................................................................................................................................3-10
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