Electromechanical Strain Mechanisms and Magnetoelectric Coupling in Polycrystalline Bismuth Ferrite

Bismuth ferrite, BiFeO3, is both a piezoelectric and multiferroic material with coupled ferroelectricity, ferroelasticity and antiferromagnetism, which can significantly improve the functional capabilities of devices. There are also other desirable properties of bismuth ferrite, including the absence of lead and high Neel and Curie temperatures well above room temperature. Additionally, it displays conductive ferroelectric domain walls that have applications in novel electronic applications. In this thesis, specialised solid state synthesis techniques were used to fabricate phase-pure polycrystalline BiFeO3 ceramics. The macroscopic dielectric and piezoelectric properties were studied and their structural origins were explored using in situ synchrotron X-ray and neutron diffraction. Firstly, the field-dependent nonlinearity of macroscopic strains under sub-coercive field cycling was studied using in situ X-ray diffraction, coupled with measurements of the macroscopic strain. This was followed by studying the Maxwell-Wagner frequency dispersion of macroscopic piezoelectric coefficient and the microscopic strain mechanism contributions via in situ sub-coercive field application. It was found that the domain wall motion and lattice strain decouple from each other. This was shown, by an analytical model, to be due to conductive domain walls. Post-poling relaxation of the macroscopic piezoelectric coefficient and domain texture was also investigated using an ex situ method and in situ synchrotron X-ray diffraction. The phenomenon is ascribed to defect dipoles in the system. Lastly, the coupling of antiferromagnetic and ferroelectric domain texture in the polycrystalline ceramic was studied by in situ neutron diffraction. Based on the results of in situ X-ray diffraction and neutron diffraction with application of electric field, the microscopic contribution to the bulk response and multiferroic order parameter coupling are studied comprehensively. These data are expected to provide valuable information for improving its piezoelectric and multiferroic properties in future studies