Defect-Phase Equilibrium and Ferroelectric Phase Transition Behavior in Non-Stoichiometric BaTiO3 Under Various Equilibrium Conditions

Open Access
Author:
LEE, SOON IL
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 08, 2006
Committee Members:
  • Clive A Randall, Committee Chair
  • Dr Zi Kui Liu, Committee Chair
  • Leslie Eric Cross, Committee Member
  • Dr Susan Trolier Mc Kinstry, Committee Member
  • Elizabeth C Dickey, Committee Member
Keywords:
  • BaTiO3
  • defect chemistry
  • ferroelectric phase transition
  • phase equilibria
  • Landau theory
  • ferroelectric properties
Abstract:
BaTiO3 has been systematically investigated with regard to the solubility region around stoichiometric BaTiO3 with a combination of techniques. This investigation uses mainly the nature of ferroelectric phase transition to define the relationships between phase equilibria, defect chemistry, and phase transition behavior in non-stoichiometric BaTiO3 under various equilibrium conditions. The phase transition temperature (TC) between the paraelectric and ferroelectric phases was varied systematically with defect concentration, which is controlled by Ba/Ti ratio, temperature, and oxygen partial pressure (PO2). These same variables give rise to changes in the latent heat of transition (∆H). In this study, the limits to the solubility of excess BaO and excess TiO2 in BaTiO3 were determined under well controlled equilibrium conditions. The resulting observation is that the different defect types (such as titanium, barium, and oxygen vacancies) all give rise to different degrees of TC variation. Under low PO2 conditions, the oxygen vacancy concentration affected the TC. Two different types of PO2 effect are noted: First, at intermediate PO2 there is ionic compensation with a constant TC dependence, and second, in the electronically compensated the extreme low PO2 region with an increase of TC. The latent heat and phase transition temperature changed systematically, indicating solid-solution on both the Ba-rich as well as Ti-rich sides of stoichiometric BaTiO3. The solubility limit was greater for the Ti-rich side. The extent of solid-solution on the Ba-rich side was much larger than previously believed. From these measurements, the solubility behavior was determined and analyzed with defect chemical reactions. Self-consistent data were obtained and successfully modeled, analytically, with defect reactions as a function of Ba/Ti, temperature, and oxygen partial pressure (PO2). Finally, Landau-Devonshire theory was used to assess the compositional dependence of the ferroelectric properties in the solid solution regimes. From the TC, ∆H, and thermal hysteresis data, higher order coefficients of the Landau phenomenological theory were determined and used to predict the spontaneous polarization, electrostriction coefficients, dielectric constants, etc. These values are in substantial agreement with previously reported data. Thus, it was shown clearly that TC and ∆H offer excellent experimental insight into the coupling between point defects and phase transition behavior. This study revised the BaO-TiO2-x phase diagram, determined the defect chemistry of BaTiO3, and provided reliable enthalpies of defect formation for the partial Schottky reactions: 2.32±0.1eV for BaO partial Schottky and 2.89±0.3eV for TiO2 partial Schottky. Also determined are the relationships between TC and defect concentration, and the compositional dependence of the Landau coefficients. This study provides a more comprehensive understanding of the interactions linking phase equilibria, defect equilibrium, and phase transition theory for the BaTiO3 system. The methodology is general, and can be applied to other systems with first order phase transitions.