Phase transitions in new lead (Pb)-free morphotropic phase boundary epitaxial ferroelectric thin films

This thesis is motivated by the recent discovery of a lead-free morphotropic phase boundary (MPB) in Bi(1-x)SmxFeO3, (BSFO) perovskite thin film system and was found to have outstanding electromechanical and functional properties at x=0.14.The thesis first discusses how a universal MPB behavior is found independent of the rare-earth (RE) dopant species in BiFeO3 (BFO) thin film system. It shows that the composition of the MPB can be tuned by controlling the average ionic size of A-site cation. Subsequent first-principles calculations suggest that the observed changes in the hysteresis loop is due to RE doping and the concomitant enhancement in the piezoelectric coefficient is because of an electric-field-induced transformation from a paraelectric orthorhombic phase to the polar rhombohedral phase.The thesis then employs comprehensive transmission electron microscopy and x-ray diffraction to unravel the structure-property correlations in the BSFO system. It is found that introducing Sm creates nanoscale antipolar clusters within the ferroelectric matrix. As the Sm3+ content increases, the antipolar clusters grow within the ferroelectric matrix, which leads to concomitant changes in local chemical environment. At the MPB composition, the FE regions are so frustrated by the surrounding anti-polar matrix, which an incommensuration phenomenon is found at the boundary between the polar and antipolar phases. This incommensuration causes the system to adopt a complex nanoscale mixture bridging between rhombohedral and orthorhombic phases. This phase-bridging structure acts as a facile platform and the most critical factor necessary to give rise to exceptional electromechanical and functional properties at the MPB.Finally, the thesis provides a complete structure-electromechanical property correlation study in RE-substituted BFO films. Piezoelectric coefficient (d33) and dielectric constant (e33) measurements along with electron diffractions reveal that the enhancement in d33 and e33 at the MPBs of the RE-substituted films (RE = Dy3+, Gd3+ and Sm3+) is due to the presence of a complex phase coexistence exhibiting lattice incommensuration. Extending the studies to the case of La3+-substituted BFO, the phase coexistence is found to be less prominent. This is attributed to the lack of increase in d33 for the La3+-substituted BFO films, in contrast to the Dy3+, Gd3+ and Sm3+-substituted films