Nye, Daniel (2017) Investigation of the structure of the perovskite system (1-x)BiFeO3-(x) (K0.5Bi0.5)TiO3. PhD thesis, University of Warwick.

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Abstract

The (1-x)BiFeO3-(x) (K0.5Bi0.5)TiO3 (BFO-KBT) system has been investigated with respect to composition and temperature. Powder samples have been synthesised by the traditional solid state method at regular intervals across the compositional range. High resolution powder x-ray diffraction measurements have been made on these powders between room temperature and up to 950ºC. The data from these measurements have been analysed by the Rietveld method, and the results of these refinements have been used to construct a phase diagram of the BFO-KBT system.

It was found that there are no sharp transitions in the phase diagram, either with composition or with respect to temperature, and that aside from the BFO end member, all samples investigated were best fitted with a mixed phase model. At low mol% KBT, it was found that the mixed phase model that best described the system was a mix of rhombohedral R3c and cubic Pm3m. As the mol% KBT was increased, the cubic phase increased, becoming dominant between 15% KBT and 20% KBT. The rhombohedral phase diminished with increasing mol% KBT and was no longer a component in the mixed phase system beyond 60% KBT. At 70% KBT, the room temperature system could still not be modelled with a single phase. A mix of cubic Pm3m and monoclinic P1m1 was used to model the system. At 90% KBT, the model was again found to be best fitted by a different mix of phases, specifically a mix of monoclinic P1m1 and tetragonal P4mm phases were used to model the data. The tetragonal phase was found to become more dominant from this point with increasing mol% KBT, but even the KBT end member was found to be best modelled with a mix of monoclinic P1m1 and tetragonal P4mm phases.

It was found that all samples become more cubic with increasing temperature. It was found that the temperature at which the change to a cubic state occurred decreased with increasing mol% KBT. It was also found that at high mol% KBT and high temperature, there was phase separation into a two-cubic mixed phase model instead of the expected single Pm3m cubic phase. This phase separation has been linked to the reported morphotropic phase boundary in the material. The BFO-KBT system was found to change little with respect to temperature below 500ºC, which makes it an attractive material for high temperature device applications if doped with a material with a stronger piezoelectric response.

In addition, single crystal x-ray diffraction of BFO-KBT crystals has been undertaken with laboratory-based and synchrotron systems. It was found that the latter was necessary to obtain good results from a refinement of the data in SHELXL due to the absorption of the BFO-KBT system, minimised through the use of a much higher energy x-ray beam. It was found in JANA2006 that the BFO-KBT system was best fit with anharmonic atomic displacement parameters, which was linked to the difference between the short range and long range order in the material.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics Q Science > QD Chemistry
Library of Congress Subject Headings (LCSH):	Perovskite -- Structure, X-ray crystallography, X-rays -- Diffraction, Bismuth compounds, Phase diagrams
Official Date:	July 2017
Dates:	Date Event July 2017 Submitted
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Thomas, Pam A.
Format of File:	pdf
Extent:	xiv, 197 leaves : illustrations, charts
Language:	eng

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