Structure-Property Relationships in Relaxor Ferroelectric Thin Films

Understanding materials structures and how they give rise to their properties defines the central mission of Materials Science and Engineering. My dissertation work is focused on understanding the structure-property relationships in epitaxial thin films made of the prototypical relaxor ferroelectric 0.68PbMg1/3Nb2/3O3-0.32PbTiO3. While much work has been done on singe crystals of0.68PbMg1/3Nb2/3O3-0.32PbTiO3, the demand for device miniaturization calls for a thorough understanding of their structure-property relationships in thin film forms. As such, the work herein is primarily based on the development of epitaxial thin films as a new platform for investigating the behavior of nanoscale-polar structures under different conditions. First, I studied the epitaxial strain dependence of domain structures and dielectric and ferroelectric properties in 0.68PbMg1/3Nb2/3O3-0.32PbTiO3 thin films using a combination of synchrotron-based X-ray diffuse-scattering, electrical measurements, and molecular-dynamics simulations. In this work, I observed that the diffuse-scattering patterns arising from the existence of polar nanodomains change from butterfly shapes to disc shapes as the epitaxial compressive strain increases due to the strain-induced polarization rotation. Even at the highest strains, the films exhibited persistent relaxor behavior due to the enhanced disorder in unit-cells with complex chemistries. After exploring the strain dependence, I studied the evolution of macroscopic phases under AC electric-fields using a synchrotron-based in operando X-ray microdiffraction technique. In this work, I observed that the polarization rotation is suppressed when the frequency of the AC electric-field becomes ultrasonic due to the limited kinetics for the growth of polar nanodomains. Building upon the understanding of strain- and electric-field-dependent structural evolutions and techniques to study them, I studied the relationship between polarization rotation and polar nanodomains using a multi-scale approach that probes the structural evolution in both the average unit-cells and polar nanodomains. I observed that the boundary between (pseudo-)rhombohedral and (pseudo-)tetragonal crystal structures of unit-cells is located in the same region of the strain-electric-field diagram as the boundary between ellipsoidal and cylindrical polar nanodomains, which are elongated along the <011> and [001], respectively. These results imply that polar nanodomains play a more direct role in polarization rotation than previously perceived and it is the strong coupling between the unit-cells and polar nanodomains that ultimately leads to the large electromechanical coupling in relaxor ferroelectrics. In addition to these, I studied the thickness-dependent evolution of polar nanodomains and various relaxor-phases using a combination of synchrotron-based X-ray diffuse-scattering, electrical measurements, and molecular-dynamics simulations. In this work, I constructed a thickness-temperature phase diagram, which was used as a guide to perform thickness- and temperature-dependent diffuse-scattering studies to probe the response of polar nanodomains to size effects. I observed that the distinguishing relaxor features of 0.68PbMg1/3Nb2/3O3-0.32PbTiO3 disappear when the thickness is below the average size of the polar nanodomains. Ultimately, this implies that the compositional heterogeneities not only determine the size of the PNDs but also play a vital role in defining the stability of relaxors itself. Lastly, I demonstrated how the advanced understanding of structure-property relationships afforded by studies presented herein can lead to significant improvements of useful materials properties. Specifically, I explored the enhancement of capacitive energy storage properties in ion-bombarded 0.68PbMg1/3Nb2/3O3-0.32PbTiO3 thin films. This work is based on the observation from the strain dependence that relaxor behavior is surprisingly persistent even in highly ferroelectric environments. The balance between persistent relaxor behavior and reduced electrical leakages in the presence of defects created by ion-bombardment with high-energy helium ions led to unprecedented capacitive energy storage performance. Overall, my work presented in this dissertation offers new understanding of structure-property relationships in relaxor ferroelectrics and demonstrates how such understanding can lead to new functionalities in old materials.

Advisor: Martin, Lane W.