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Title:The Freezing Mechanisms of Perovskite Relaxor Ferroelectrics and the Thermodynamic Stability of their Ferroelectric States
Author(s):Delgado, Matthew E.
Director of Research:Weissman, Michael B.
Doctoral Committee Chair(s):Stack, John D.
Doctoral Committee Member(s):Weissman, Michael B.; Dahmen, Karin A.; Hubler, Alfred W.
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Relaxor Ferroelectrics
Abstract:Relaxor ferroelectrics are disordered crystalline materials that have short-range ferroelectric order, known as polar nanoregions. These materials do not exhibit a ferroelectric phase transition at zero field, but rather enter a glassy-like relaxor regime. The source of the glassy freezing and the thermodynamically stable phase in the relaxor regime are still uncertain. This thesis attempts to explain the entities and interactions involved in the emergence of glass-like behavior, and to describe their thermodynamically stable phase at low temperature. To do this, we conducted dielectric susceptibility and pyroelectric current measurements on two prototypical perovskite relaxors, PbMg1/3Nb2/3O3 (PMN) and the solid solution of PMN with the ferroelectric PbTiO3 (PT), (PbMg1/3Nb2/3O3 )1-x(PbTiO3 )x (x = 12%). To understand the onset of glassy freezing, we analyzed the effect of a DC external field Edc on the Vogel-Fulcher freezing temperature TVF. We found a reduction in TVF when the external field along the [111] direction was increased. This implied that the interactions among the polar nanoregions are responsible for the onset of glassy freezing. The effect of Edc applied along [100] direction on TVF suggested the key interactions are a mixture of dipolar and quadrupolar strain interactions. Furthermore, in PMN we found there exist a field induced two-step phase transition into the ferroelectric state and two-step depolarization on heating. We were able to identify the first step on cooling with the second step transition (i.e. higher temperature) on heating. The second step transition on heating looks like a possible phase transition. Moreover, the two steps in depolarization showed exhibited different stabilities. The ferroelectric state associated with the low temperature step lost polarization more easily when aged below the depolarization peak than the state associated with the second step. This suggested that the thermodynamically stable phase in the relaxor regime is a more disordered phase. Additionally, we performed aging experiments on PMN-12%PT in the relaxor regime. We found that even after the sample was highly polarized spinglass-like aging remained. This implied the need for more than one order parameter to describe the low temperature glassy phase.
Issue Date:2010-01-06
Rights Information:Copyright 2009 Matthew E. Delgado
Date Available in IDEALS:2010-01-06
Date Deposited:December 2

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