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Title: Structure-property relationships in oxide ion conducting hexagonal perovskites
Author: McCombie, Kirstie S.
ISNI:       0000 0004 8498 4974
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2019
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Research has been driven into the development of electrolyte materials for solid oxide fuel cells which exhibit high ionic conductivities at intermediate temperatures. Ba3MoNbO8.5 is the first hexagonal perovskite derivative reported to exhibit oxide ion conductivities comparable to that of conventional electrolytes. Ba3MoNbO8.5 crystallises in a structure intermediate between the 9R hexagonal perovskite and palmierite structures, containing a mixture of octahedral and tetrahedral geometry, with intrinsic oxygen vacancies. The oxide ionic conductivity exhibited by Ba3MoNbO8.5 was found to increase as the ratio of tetrahedra to octahedra within the structure increases at greater temperatures. The properties of the tungsten analogue Ba3WNbO8.5 were investigated and it was found that the substitution of W6+ for Mo6+ increases the pO2 range over which oxide ion conduction is observed, with negligible detriment to the bulk conductivity at 600 °C. Results from neutron diffraction showed that the percentage of tetrahedra within the average structure of Ba3WNbO8.5 is significantly lower than that in Ba3MoNbO8.5, resulting in lower bulk conductivities exhibited by Ba3WNbO8.5 at lower temperatures. Variable temperature neutron diffraction experiments confirmed that a similar structural modification that observed in Ba3MoNbO8.5 also occurs in Ba3W1.2Nb0.8O8.6. Above 300 °C, the ratio of tetrahedra to octahedra in the structure increases, and at a faster rate than Ba3MoNbO8.5, enhancing the oxide ion conductivity so that by 600 °C, the oxide ion conductivity of the two compounds are comparable. The solid solution Ba3MoNb1-xVxO8.5 was synthesised to investigate the effects of vanadium doping on the compound Ba3MoNbO8.5. Results from neutron diffraction have demonstrated unusual behaviour in the variation of the lattice parameter c and have shown that the ratio of tetrahedra to octahedra within the average structure increases with vanadium doping.
Supervisor: Mclaughlin, Abbie ; Skakle, Jan M. S. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Solid oxide fuel cells ; Perovskite