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Title: Novel nitrides and chalcogenides with the potential for superconductivity
Author: Campbell, Holly Jane
ISNI:       0000 0004 8498 689X
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2019
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The primary aim of the work in this thesis was to design, synthesise and characterise novel superconducting materials. Inspiration for the studied materials came from information learned about conventional BCS theory materials and the high-temperature superconductors. Li3-xFexN was investigated as a potential superconductor in analogy to the known superconducting iron pnictide materials LiFeAs and LiFeP. Two methods of Li3-xFexN synthesis were studied: thermal decomposition of Li3FeN2 under Ar gas flow, and a direct synthesis between Li3N and Fe under Ar gas flow. Thermal decomposition was observed to occur in 2 steps, leading to an intermediate Li2.4Fe0.6N product, before complete decomposition to the elements. Direct synthesis was complete in one step, giving the Li2.65Fe0.35N product. The structure of Li3-xFexN is different from the pnictide superconductor analogues LiFeAs and LiFeP due to differences in the bonding arrangement between Fe and N or As/P and because Fe concentration in Li3-xFexN has never been found to approach x = 1 in previously produced samples, leading to a disordering of both Li at the (1) position and Fe. The aim of this work was to measure the properties of a sample with a higher Fe concentration than measured in literature. The band gap of Li2.65Fe0.35N was inferred to be indirect and in the IR range. In the presence of an applied magnetic field, Li2.65Fe0.35N exhibited a ferromagnetic transition below 127 K. In the absence of an applied field, Li2.65Fe0.35N behaved as a spin-glass below 105 K because of the Li(1)/Fe disorder. Both the band gap and magnetic measurements were consistent with literature on samples with slightly lower Fe concentrations. LiPdNx was successfully synthesised for the first time via a reaction between Li3N and Pd under N2 gas flow. LiPdNx was observed to adopt the hexagonal LiPd alloy structure, with N occupying trigonal prismatic positions between the Li and Pd layers. The reaction between Li3N and Pd involved 2 N2 gas loss steps, resulting in the LiPdN0.14 composition in the product. DR UV-vis suggested that LiPdNx was metallic, without a band gap. Magnetic measurements did not indicate signs of superconductivity, but Curie-Weiss law paramagnetism was observed. BaZrN2 was synthesised via a reaction between Ba2N and ZrN under both vacuum and N2 atmospheres and was observed to exhibit superconductivity at 9.2 K. Previous studies had suggested that the superconductivity that emerged arose from ZrN impurities within samples; no such impurity was found in the BaZrN2 sample via PXRD, but observed Pauli paramagnetism in SQUID measurements suggested that ZrN was indeed present, thus it was concluded that superconductivity was intrinsic to the binary nitride. The original plan was to make thin-films of BaZrN2, however this was not possible within the given timeframe. CuxSnSe was synthesised for the first time in a multi-mode microwave reactor ever via a solid-state reaction between the elemental powders. Cu was found to occupy spaces between Sn and Se within the SnSe layers, and cell parameter changes with increased doping concentration agreed with this observation. The calculated values for SnSe band gaps were consistent with literature, and the band gap of CuxSnSe increased with doping concentration. This suggested that a Moss-Burstein shift may have been introduced to the SnSe density of states upon Cu doping. Superconductivity attributed to CuxSnSe was not observed, and it was concluded that the material displayed Curie-Weiss law paramagnetic behaviour. Samples of CuxBi2Se3 and CuxBi2Te3 were synthesised in bulk powder form for the first time ever using a solid-state microwave reaction between the elemental powders. Cu was observed to occupy octahedral positions between the Bi2Ch3 blocks, which is in agreement with literature. Band gap values of Bi2Se3 and Bi2Te3 decreased upon intercalation with Cu, which was thought to be a factor in the transition from topological insulator parent materials to topological superconductor intercalated materials. SQUID measurements indicated superconductivity below 3 K in the synthesised CuxBi2Se3 and CuxBi2Te3 samples, meaning that the microwave solid-state reactions were successful in production of topological superconductors.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Q Science (General) ; QC Physics ; QD Chemistry