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Title: Synthesis and characterisation of copper oxide superconductors and related materials
Author: Chapman, J. P.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1997
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This thesis is concerned with the synthesis of high critical temperature (Tc) copper oxide superconductors and related materials. Details are given of a range of substituted phases based on the TBa2Cu3O7 structure. Chapter 1 introduces the discovery and development of the high Tc superconductors and describes their structural features. Theoretical considerations of the methods of structure determination are presented in Chapter 2 and the experimental techniques are detailed in Chapter 3. Chapter 4 gives the results of a study into the structural properties of borate substituted phases. The upper limit (x) of boron substitution in the YSr2Cu3-xBxO7 structure was found to be ≈ 0.75. Rietveld refinement using X-ray diffraction data shows YSr2Cu2.25B0.75O7 to be orthorhombic (a = 3.8176(2)Å, b = 3.8345Å, c = 10.9708Å). An electron microscopy study of this material showed a 4B:Cu ordering, giving an a x 5b x 2c supercell, consistent with an upper limit of x ≈ 0.8. Attempts to Rietveld refine the geometry of the borate group from neutron diffraction data were unsuccessful. Exploration of several mixed T1 or Bi copper oxoborate compositions did not yield any new phases. The effects of cation substitutions and inherent disorder in T1Sr2(Ca,Y)Cu2O7 based materials are described in Chapter 5. 'T1Sr2(Ca0.4Y0.6)Cu2O7' was found, by resonant synchrotron X-ray diffraction, to be (T10.922(4)Cu0.078)Sr2(Ca0.195(5)Y0.805)Cu2O7, with the inherent cross-substitution giving an optimally doped material with an average copper oxidation state of +2.14 and Tc = 86K. A comparison of first row transition metal substituents in this structure shows Mn to be a particularly good substituent, yielding an almost phase pure material with a high Tc (97K). Chapter 6 describes attempts to make the Mn analogue of YBa2Cu3O7, YBa2Mn3O9-x, which led to the discovery of a new tetragonal layered Mn oxide, YBaMn2O5 (a = 3.9204Å, c = 7.6614Å). This material exhibits no oxygen non-stoichiometry and is a valence ordered, insulating ferrimagnet below 167K.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available