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Title: Structure, properties, and chemistry of iron-based superconductors
Author: Cassidy, Simon J.
ISNI:       0000 0004 6062 515X
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2015
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This thesis reports the synthesis and characterisation of several layered pnictides, chalcogenides, and oxychalcogenides, with an emphasis on materials that exhibit high temperature superconductivity. High and low temperature techniques have been used to synthesise new materials and modify their properties. The majority of this work has been focused on the synthesis of superconducting materials with the general formula Ax(NH3)y(NH2)zFeSe, where A is an alkali metal. These materials are formed from a co-intercalation of alkali metals, ammonia, and alkali metal amides into the interlamellar space of pre-formed tetragonal FeSe. There is a remarkable increase in Tc associated with this intercalation, from 8 K in FeSe to a maximum of 46 K in the products. A range of characterisation techniques including neutron and X-ray diffraction, EXAFS, and SQUID magnetometry have been used to identify a variety of crystal structures, compositions, and properties adopted by these materials. The synthesis procedure of these materials where, A = Na and K, has been studied in-situ via powder X-ray diffraction at world-class central facilities, revealing new phases, intermediates, and activation energies. The Kx(NH3)y(NH2)zFeSe phases are found to undergo a topotactic decomposition step to become Kx'Fe2-y'Se2 phases on annealing, which has also been studied by in-situ powder X-ray diffraction. Additional studies on Na1-xFe2-yAs2 and CaFeSeO have been performed. Na1-xFe2-yAs2 is the product of a room temperature deintercalation of sodium and iron from NaFeAs, which changes the superconducting properties of the material. XAFS measurements have been used to characterise the local structure of the materials, which supports the conclusion that iron is deintercalated from the parent material and gives new insight into the effect of the iron vacancies on the local structure. CaFeSeO is a newly discovered material that adopts a never-before-seen crystal structure, which has been solved from powder X-ray diffraction data. Intricate vacancy ordering exists in the material, which contributes to a peculiar mixture of magnetic behaviours including signatures of a spin glass, ordered antiferromagnet, and an ordered ferromagnetic component. All of these behaviours however, can be rationalised by the nuclear and magnetic structure of the material that have been refined using powder neutron diffraction.
Supervisor: Clarke, Simon Sponsor: Engineering and Physical Sciences Research Council ; Diamond Light Source
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available