Electric Field-Induced Phase Transitions in Ferroelectrics at Polymorphic Phase Boundaries

IAMSASRI, THANAKORN. Electric Field-Induced Phase Transitions in Ferroelectrics at Polymorphic Phase Boundaries. (Under the direction of Jacob L. Jones.) Ferroelectric and dielectric materials are used in many applications, including capacitors, actuators, and energy harvesting. In general, the piezoelectric and dielectric properties of these materials reach a maximum at the morphotropic or polymorphic phase boundary, which lie between two different phases of the same structure type. At the phase boundary, the two coexisting phases have similar free energies. By applying external stimuli such as pressures or electric fields, the free energies of two phases can be changed, resulting in an induced phase transition. Electric field-induced phase transitions in ferroelectrics have been observed using transmission electron microscopy (TEM) and X-ray diffraction (XRD). However, there are limited studies on the field-induced phase transitions of ferroelectrics because it requires a characterization technique that can probe structural evolution under electric fields. This study investigates the field-induced phase transitions of ferroelectrics and dielectrics using a combination of synchrotron XRD techniques including in situ XRD under electric fields, high resolution XRD, and time-resolved XRD. This combination of characterization techniques allows one to observe a field-induced phase transition and also quantify it using the intensities and positions of peaks from XRD patterns. Two different material systems are investigated in this study: Li-modified Na0.5K0. 5NbO3 (LNKN) and BaTiO3-BiZn0.5Ti0. 5O3 (BT-BZT). LNKN is a ferroelectric, and BT-BZT is a relaxorferroelectric (i.e. exhibits frequency dependence of dielectric permittivity). For both LNKN and BT-BZT, the field-induced phase transition was observed only in compositions located at the polymorphic phase boundary, but never in single-phase compositions. This result shows that the field-induced phase transition is therefore closely related to the high piezoelectric properties at the phase boundary. Additionally, domain reorientation in single-phase orthorhombic LNKN was studied using in situ XRD under electric fields. For the first time, an equation to quantify domain reorientation in orthorhombic lattice was developed and applied to LNKN. In prior studies, equations had only been developed to quantify the degree of domain reorientation for tetragonal and rhombohedral lattices. The results from this work show that the degree of domain reorientation increases as the composition approaches the orthorhombic-tetragonal phase boundary. Moreover, the time and frequency dependence of field-induced phase transition in BTBZT was studied using time-resolved XRD. The field-induced phase transition in BT-BZT strongly depends on the frequency of the applied fields, which corresponds with the relaxor behaviors observed in the macroscopic properties such as polarization and strain as a function of electric field. The kinetics of the field-induced phase transition in BT-BZT were modeled by a modified Kolmogorov-Avrami-Ishibashi (KAI) equation. In contrast to prior works, this study utilized a Bayesian inference method to obtain the distribution of parameters in the modified KAI equation without additional assumptions on the distribution. Bayesian inference allows for more variety in distributions (e.g. asymmetry and multimodality), whereas prior works usually assumed the distribution to be Gaussian. In summary, the results from this research improve the current understanding of fieldinduced phase transition in ferroelectrics and relaxors. Specifically, it explores the structural evolution of ferroelectrics during the application of high electric fields field amplitudes that are typically seen during the poling process. Since the poling process directly affects the piezoelectric properties, the study of field-induced phase transitions is essential for understanding the origins of enhanced properties at phase boundaries, which are important for engineering devices with desired properties.