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Title: Structure-property relations in rare earth doped BiFeO3
Author: Karimi, Sarah
ISNI:       0000 0004 2720 9023
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2012
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RE-doped BiFe03 (RE = La, Nd, Srn, Gd) compositions have been produced and investigated with a view to establishing a broad overview of their crystal chemistry and domain structure. For less than 10% RE dopant, the perovskite phase in all compositions could be indexed according to the rhombohedral, R3c cell of BiFe03. For La, Nd and Srn doped compositions with more than 15% dopant, a new antipolar phase was stabilised similar in structure to PbZr03 Pbam symmetry with a -V2ao2-V2ac,4ac unit cell where a, is the psuedocubic unit cell. The orthoferrite, Pnma structure was present in all RE-doped BiFe03 compounds with higher dopant concentration. The compositional window over which the PbZrO3-like phase was stable increased with increasing end-member tolerance factor, t (effectively controlled by the RE ionic radius). On heating, the PbZrO3-like phase transformed to the orthoferrite, Pnma structure. Tc for all compositions decreased with decreasing A-site, average ionic polarizabilty and end member tolerance factor. For compositions with R3c symmetry, superstructure and orientational and translational (anti-phase) domains were observed in a manner typical of an anti-phase tilted, ferroelectric perovskite. For the new PbZrO3-like phase orientational domains were observed along with antiphase boundaries associated with quadrupling of the unit cell due to the antipolar displacements of the Bi/RE-ions Neutron powder diffraction was used to determine changes in the nuclear and magnetic structures across the phase transitions that relate the high-temperature non- polar (Pnma) to the low temperature polar (R3c) and anti-polar (Pbam) structures, respectively. The high-temperature Pnma phase has a -√2ac, --√2ac,2ac cell with an aa c + octahedral tilting. The low-temperature polar R3c structure is similar to the B-phase of pure BiFe03 and is rhombohedral with aaa octahedral rotations and cation displacements along the peudocubic [111] direction. Although electron diffraction revealed weak additional doubling along c (denoted by the appearance of ¼ {001} in addition to ~{11O} reflections) of the PbZr03 cell, Neutron and X-ray diffraction could not reliably refine a larger cell. This is attributed to the weak intensity of the 1/4 {OO I} reflections and the short coherence length of the 4ac modulation in the c- direction. The transitions from the high temperature Pnma phase to both low temperature phases are accompanied by a large discontinuous expansion of the lattice volume in the low- temperature structure. The Pnma to R3c transition occurs in the paramagnetic state and generates no detectable changes in the magnetic structure. In contrast, the Pnma to Pbam transition, which occurs in the magnetic state, is accompanied by abrupt 90 ° reorientation of the magnetic dipoles. Coupling between the nuclear and magnetic structures is manifested in a significant magnetization anomaly. The dielectric properties were studied as a function of temperature but no anomalies were recorded that were coincident with the structural phase transitions observed by differential scanning calorimetry. The absence of peaks in permittivity at T c was attributed to high conductivity with space charge polarisation dominating the dielectric response.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available