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Title: The structures and properties of layered pnictides and oxychalcogenides
Author: Pitcher, Michael J.
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2011
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This work focuses principally on two compounds, CeCu1-xOS and LiFeAs, which have related layered structures but exhibit radically different physical properties. The nature of the air sensitivity of the ZrCuSiAs-type oxysulfide CeCu1-xOS has been investigated by neutron diffraction and magnetometry. It was found that this compound can be made fully stoichiometric, with structural and magnetic properties that are consistent with other LnCuOS compounds, indicating that this is a bona-fide Ce3+ compound. Upon air exposure, Cu ions are extruded from the sulfide layer to leave a Cu-deficient phase with contracted unit cell parameters and a diminished paramagnetic moment consistent with mixed-valence Ce3+/4+.The extruded Cu forms CuO and can be re-inserted into the sulfide layer by heating under a reducing atmosphere. This explains the anomalous behaviour of CeCuOS reported throughout the literature and has implications for the behaviour of other layered Cu-sulfides with oxidisable cations. At low temperatures Cu-deficient CeCu0.8OS was found to exhibit structural ordering of Cu+ ions and vacancies, resulting in a √5a x √5a basal expansion of the high-temperature unit cell. The layered iron arsenide LiFeAs was synthesised and found to be superconducting below 17 K. Joint XRD/NPD measurements showed unambiguously that the compound adopts the anti-PbFCl structure with Li ions in a square-pyramidal LiAs5 environment. No evidence was found for an orthorhombic structural distortion at low temperatures. Further diffraction experiments showed that the compound can be made with non-stoichiometric compositions Li1-yFe1-y for small values of y (<0.05), as Fe can be accommodated on the Li site. This type of non-stoichiometry was found to strongly inhibit superconductivity (which was quenched entirely when y>0.02). Three series of compounds of type LiFe1-xMxAs (M = Mn, Co, Ni) were synthesised and characterised struturally bu high-resolution XRD and/or NPD. Substitution by Co and Ni was found to cause a monotonic decrease in Tc, and Ni was found to be twice as effective at suppressing Tc as Co. MuSR measurements showed the penetration depth increasing with Co and Ni substitution, consisitent with the superconducting state becoming less robust. Substitution by Mn was found to strongly inhibit superconductivity, and this behaviour is reminiscent of the non-stoichiometric Li1-yFe1-yAs materials. The structures and superconducting properties of LiFeAs and NaFeAs were studied under high pressures. Equations of state were obtained for each compound. Hydrostatic pressure was found to distort of the FeAs4 away from ideal tetrahedral geometry in both compounds. These changes caused a monotonic decrease in Tc in LiFeAs, but has a smaller and more complex effect on the Tc of NaFeAs. Furthermore, NaFeAs was found to undergo a structural transition above P = 22 GPa, but the high-pressure structure could not be solved and this will become a target for future work.
Supervisor: Clarke, Simon J. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Chemistry & allied sciences ; Crystallography ; High-Temperature Superconductivity ; Inorganic chemistry ; Solid state chemistry ; Superconductivity ; Oxychalcogenide ; Pnictide ; Air sensitive solids