Seonkyu Shin
(Advisor: Prof. Nazanin Bassiri-Gharb)

will propose a doctoral thesis entitled,

Antiferroelectricity of the PbHfO3–PbZrO3 Thin Films

On

Tuesday, October 13 at 2:00 p.m.
J. Erskine Love Room 311

or Virtually via MS TEAMS 

Abstract

Antiferroelectrics are materials in which individual unit cells possess spontaneous polarization, but antiparallel alignment of dipoles in adjacent unit cells results in zero net polarization macroscopically Under a sufficiently high external electric field, the material undergoes a field-induced phase transition from the antipolar state to a polar state, where dipoles can be reoriented along crystallographically allowed direction(s) better aligned with the applied electric field. The antipolar state, however, can be restored upon electric field removal. The field-induced transitions (antipolar-to-polar at Ef and polar-to-antipolar at Ea) are accompanied by large nonlinear and hysteretic changes in dielectric, electro-optic, electrocaloric, and electromechanical responses. These characteristics make them attractive for various functional applications such as high-power (pulsed) capacitors, optical attenuators, phase shifters, solid-state refrigerants, and high force-high displacement actuators.

In 1951, PbZrO3 was the first material to be reported to exhibit antiferroelectricity, and has since been used extensively as the end member of Pb(Zr1-xTix)O3 (PZT) ceramics. PbZrO3 has thus been the archetypal antiferroelectric and a platform for evaluating antiferroelectric phenomena. However, the antiferroelectric nature of PbZrO3 has been the subject of recent literature debate. Anisotropic weak ferroelectricity and modulated (rather than simple antiparallel) Pb displacement observed in multiple experimental studies have raised the possibility that PbZrO3 may be ferrielectric rather than truly antiferroelectric.

PbHfO3 is another antiferroelectric perovskite oxide. Because PbHfO3 is isostructural with PbZrO3, the possibility that PbHfO3 may also be ferrielectric remains an open question. However, despite the first report of antiferroelectricity in PbHfO3 in 1953, systematic studies of polar response in this material remained relatively limited until the early 2000s. The relatively high Ef of bulk ceramic and single-crystal PbHfO3 compared to their PbZrO3 counterparts complicates experimental access to its field-induced phase transition, as dielectric breakdown could occur before complete switching is achieved. Moreover, Pb(HfxTi1-x)O3 did not exhibit the exceptional ferroelectric properties observed in the PZT system. However, since the mid-2000s, advances in processing techniques have enabled the fabrication of high-quality thin films, leading to renewed interest in PbHfO3 in thin film form. This study aims to characterize the antiferroelectricity of PbHfO3 thin films and investigate phase stability and field-induced phase transitions in PbHfO3-PbZrO3 thin films. 

The first phase of this study will center on the chemical solution processing and functional characterization of PbHfO3 thin films, pursuing processing-structure-property correlations. PbHfO3 thin films with different preferred orientations are obtained; 84% 021O-oriented and 86% 001O-oriented PbHfO3 thin films are used to investigate the antiferroelectricity of PbHfO3 thin films. (The ”O” subscript underlines the use of the orthorhombic unit cell reference for the crystallographic directions) Functional properties of PbHfO3 are analyzed to compare anisotropic phase stability and field-induced phase transition behavior. To further understand the phase stability and field-induced transition behavior of PbHfO3 thin films, we examine the kinetic stability through frequency dependent measurements. The relative stability of the antipolar and polar phases and the stability of the reversible field-induced phase transition will be evaluated through temperature dependent measurements and fatigue testing, respectively. 

Building upon the understanding of antiferroelectricity in PbHfO3 thin films, we will extend this study to PbHfO3/PbZrO3 multi-layered thin films to elucidate the influence of B-site cation distribution on antiferroelectric behavior. The phase stability of the two materials is expected to be modified starting from the interfaces. Additionally, electric field redistribution, lattice mismatch-induced strain, and defect densities may vary depending on the distribution of B-site cations, thereby influencing phase stability and field-induced phase transitions. This study will progress to PbHfO3–PbZrO3 solid-solution thin films to determine the relative stability of antipolar and polar phases and the nature of the field-induced phase transitions within a homogeneous system. Comparison with heterogeneous multi-layered thin films will clarify whether the trends in phase stability and field-induced phase transitions associated with B-site cation distribution persist in the homogeneous solid- solution limit. Ultimately, we aim to establish a composition- and electric field-dependent phase diagram of the PbHfO3-PbZrO3 solid-solution system in thin film form.

Committee
    • Prof. Nazanin Bassiri-Gharb – George W. Woodruff School of Mechanical Engineering (Advisor)
    • Prof. Juan-Pablo Correa-Baena – School of Materials Science and Engineering

    • Prof. Rosario Gerhardt – School of Materials Science and Engineering

    • Prof. Eric Vogel – School of Materials Science and Engineering 

    • Prof. Asif Khan – School of Electrical and Computer Engineering