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Title: Structures and properties of perovskites and pyrochlores from hydrothermal synthesis
Author: Daniels, Luke M.
ISNI:       0000 0004 5924 1132
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2015
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The hydrothermal syntheses of new complex pyrochlore and perovskite oxide materials have been explored. Disorder is prevalent in these materials, so their structures and properties were investigated using a wide range of techniques. New pyrochlore oxides with compositions (Na0:60Bi1:40)(Fe1:06Mn0:17Bi0:77)O6:87 and (K0:24Bi1:51)(Fe1:07Mn0:15Bi0:78)O6:86 were hydrothermally produced in solutions of NaOH or KOH, respectively, at 200oC for 6 hours. These pyrochlores are metastable, decomposing at ca 400oC, suggesting their formation would be impossible through conventional solid state synthesis. Their average structures were studied and are described by Fd3m symmetry, with considerable disorder present in the A2O0 network. Investigation of their local structures through total scattering confirms the extent of disorder within these materials, showing significant deviation from the average structure. RMC was used to model accurately this disorder; resulting in interesting atomic arrangements for the A and O0 sites. The presence of Fe3+ and Mn4+ within these disordered structures leads to spin-glass type magnetic behaviour, with no observed long-range ordering interactions, typical of geometrically frustrated pyrochlores. A new mixed rare-earth chromite LaxSm1{xCrO3 perovskite solid solution (x = 0.0, 0.25, 0.5, 0.625, 0.75, 0.875, and 1.0) was prepared by single-step hydrothermal reaction at 370oC for 6 hours. The distorted perovskite structures are described by orthorhombic space group Pnma, whilst XRD and Raman scattering confirms the increasing structural distortion towards SmCrO3. The hydrothermal treatment results in dendritic-type crystallites, shown by STEM, offering greater homogeneity of particle size over solid state methods. Magnetometry reveals the onset of antiferromagnetic ordering of Cr3+ spins below room temperature, with magnetic interactions below these temperatures greatly dependent upon the structural distortion and samarium content. Spin-phonon coupling effects and switchable polarisations were observed at temperatures corresponding to magnetic events, suggesting these phenomena are spontaneously induced by magnetic interactions. A study of A site cation size variance effects in mixed rare-earth orthochromites was performed on Y0:5Ho0:5CrO3, La0:5Sm0:5CrO3, and La0:5Tb0:5CrO3 solid solutions, prepared through both hydrothermal and solid state methods. Increased variance in the La-Tb system resulted in separation of the two rare-earths in local layers, observed by atomically-resolved EELS, whilst attempts to make La-Ho solid solutions were not possible as the A site radius variance was too great. Separate rare-earth regions were observed in the solid state sample on the nano-scale, showing that greater homogeneity was achieved through hydrothermal synthesis. The homogeneity of rare-earth cation mixing also in uenced the magnetic properties of these materials, with dramatic changes being observed upon annealing the layered hydrothermal material.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry