Average and local structure, phase transitions and physical properties of key lead-free ferroelectric materials

This thesis presents a comprehensive structural and temperature-dependent properties study of two key lead-free ferroelectric families of materials: the bismuth ferrites and the alkaline niobates. Doped bismuth ferrites are of interest not only as lead-free ferroelectric materials but also as multi-ferroic materials. Both alkaline earth doped Bi^{III}{u2081}-xA^{II}xFe^{III}O{u2083}-x/{u2082} and rare earth doped Bi^{III}{u2081}-xLn^{III}xFe^{III}O{u2083} systems are investigated. An incommensurately modulated, perovskite-related, solid solution is found to be characteristic of both the Bi^{III}{u2081}-xA^{II}xFe^{III}O{u2083}-x/{u2082}, A = Sr and Ca systems. Contrary to previous reports, a tetragonal distortion is found across the phase diagram. Despite prior expectations, piezoresponse force microscopy suggests that BCFO is non-piezoelectric at room temperature. Through transmission electron microscopy studies, Mossbauer spectroscopy as well as temperature dependent electrical and mechanical examination, the incommensurate modulation in these systems is attributed to oxygen vacancy and associated alkaline earth ordering. A characteristic relaxation in the dielectric spectroscopy results is assigned to oxygen vacancy hopping while the material as a whole is found to be semiconducting. Materials of composition Bi^{III}{u2081}-xLn^{III}xFe^{III}O{u2083}, Ln = Nd and Sm, are synthesised by both solid state and sol-gel synthesis techniques. Magnetization and electrical measurements show that BSFO 10 and BNFO 10 are simultaneously ferroelectric and ferromagnetic. Correlated temperature dependent magnetic, electrical and mechanical behaviours were investigated, with similar low temperature magnetic and mechanical anomalies in both materials. Neutron diffraction is used to investigate the Neel temperatures of these materials. Finally, the K^{I}xNa^{I}{u2081}-xNb^{V}O{u2083} (KNN x) system is systematically investigated in two ways. Firstly, the KNN system is examined with regard to its ferroelectric and piezoelectric properties, primarily near x = 35% (KNN 35), with and without y% Ta(V) doping (KNNT 35/y%). Ta doping of KNN 35 is found to improve piezoelectric behaviour, decrease grain size, decrease elastic stiffness and to lower high temperature phase transitions. The KNN 35 region of the phase diagram is found to be remarkably similar to the more often characterised K{u2080}.{u2085}Na{u2080}.{u2085}NbO{u2083}{u03B6} region. The reported morphotropic phase boundary related property enhancements in the KNN x system are also discussed in light of the results obtained. Secondly, the KNN x system is examined with regard to structural disorder across the entire phase diagram. Electron diffraction shows transverse polarized planes of diffuse intensity perpendicular to the parent perovskite [010] axis across the KNN x phase diagram at room temperature, demonstrating 1-D ferroelectric disorder. Additionally, 1-D rods of diffuse intensity of the type G +/- {u03BE}, 1/2, 1/2]p* and G +/- [1/2, 1/2, {u03BE}]p* (G a parent perovskite sub-structure reflection) are also observed, demonstrating octahedral tilt disorder at all compositions. Bond valence sum calculations suggest that this octahedral tilt disorder is due to the crystal chemical conflict between the larger K ions and the smaller Na ions occupying the perovskite A site. The reported unit cells and space groups of the phases in KNN x are re-examined in light of the inherent ferroic and octahedral tilt disorder characteristic of this system. The thesis is concluded with a summary of the key findings and some propositions for further work in these systems. -- provided by Candidate