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Title: Topochemical reduction and doping of some complex iridium oxides
Author: Page, Jacob
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2018
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The main body of the work is focussed on the topochemical reduction of iridium containing oxides which adopt the perovskite or the n = 1 Ruddlesden-Popper structures. The second part of this work is focussed on the doping of Sr2FeIrO6. The topochemical reduction chemistry of some 3d/Ir containing oxides was investigated using the group I/II hydrides as reducing agents. These reactions were successful however the by-products of reaction could not be removed without decomposing the reduced phases. To overcome these issues new reduction methods were developed. Reduction of the double perovskite Sr2FeIrO6 with 5%/95% H2/N2 resulted in the formation of Sr2FeIrO4. At temperatures above 115 K Sr2FeIrO4 adopts a highly symmetric infinite-layer structure that consists of a rigorously ordered array of vertex sharing IrIIO4 and FeIIO4 units. The structure distorts with the onset of magnetic order. At above 115 K the spins in Sr2FeIrO4 are strongly interacting but frustrated. At 115 K the spins in Sr2FeIrO4 order resulting in a canted-antiferromagnetic state that is highly unusual and is dominated by Ir-Ir and Fe-Fe exchange interactions. To extend this work we synthesised and reduced Sr2Fe0.5Ir0.5O4. Sr2Fe0.5Ir0.5O4 adopts the archetypal n=1 Ruddlesden-Popper structure. Magnetisation data suggest Sr2Fe0.5Ir0.5O4 may have two magnetic transitions however the structure is paracrystalline so the magnetic structures cannot be determined. Reduction of Sr2Fe0.5Ir0.5O4 using a zirconium getter resulted in the formation of Sr2Fe0.5Ir0.5O3. The structure of Sr2Fe0.5Ir0.5O3 is the layered analogue of the infinite-layer structure. The reduction of the solid-solution CaMn0.5Ir0.5O3 was achieved using a NaH getter reaction and resulted in the formation of CaMn0.5Ir0.5O2.5. The B-site disorder present in CaMn0.5Ir0.5O3 is retained in CaMn0.5Ir0.5O2.5 and the oxide vacancies introduced are also disordered. CaMn0.5Ir0.5O2.5 is the first example of IrIII in a topochemically reduced phase and the first example of square pyramidal IrIII. The final set of phases that were synthesised and reduced were LaxSr2-xCo0.5Ir0.5O4 (x = 0, 0.5). These n=1 Ruddlesden-Popper phases were reduced using a zirconium getter reaction to form LaxSr2-xCo0.5Ir0.5O3 (x = 0, 0.5). Similarly other Ruddlesden-Popper phases and reduced analogues, LaxSr2-xCo0.5Ir0.5O3 (x = 0, 0.5) are paracrystalline so accurate structural determination is difficult however refinements suggest these phases could have a distorted anion structure. The doping chemistry of Sr2FeIrO6 was investigated by doping the double perovskite with low levels of Ca, Ba, Ga and La. Structural analysis showed that the doped samples undergo phase separation and consist of an I1̅ phase and a P21/n phase. Magnetisation data suggest that the doped samples have two magnetic transitions. The second transition is to a type-II antiferromagnetically ordered state similar to that of Sr2FeIrO6. It is not clear what causes the first transition but Fe Mössbauer data show it is not due to Fe-based spins, indicating a jeff ≠ 0state for IrV.
Supervisor: Hayward, Michael Sponsor: Leverhume Society
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available