| Summary: | Understanding the effects of external stimuli on electronic and ionic defects in functional oxides is the key to design next generation energy conversion/storage and memory devices. Mechanical strain and high electric fields are known to commonly exist in thin film functional oxides and significantly affect the electronic and ionic defect concentrations, reaction energy landscapes, and transport properties. This thesis focuses on quantifying the influence of external stimuli such as strain and electric field on the defect chemistry in functional oxides, including perovskites ABO₃ and solid oxide fuel cell materials.
We first quantify the nonlinear dielectric response of neutral oxygen vacancies, composed of strongly localized electrons at an oxygen vacancy site, in perovskite oxides of the form ABO₃. Our approach implements a computationally-efficient local Hubbard U correction in density functional theory simulations. These calculations indicate that the oxygen vacancy dipole moment correlates strongly with B site cation reducibility and lattice volume. Next, we select SrTiO₃ as our prototypical perovskite and assess the effects of biaxial strain on the stability of electronic defects at finite temperature. We constructed a predominance diagram for free electrons and small electron polarons in this material as a function of biaxial strain and temperature. We found that biaxial tensile strain in SrTiO₃ can stabilize the small polaron, leading to thermally-activated and slower electronic transport consistent with prior experimental observations on this material. These findings also resolved apparent conflicts between prior atomistic simulations and conductivity experiments for biaxially strained SrTiO₃ thin films.
To validate our prediction and to resolve the controversy from earlier studies on quantifying the strain-induced ionic conductivity enhancement, we develop an experimental platform that facilitates in situ application of tunable in-plane strain and concurrent impedance measurement on one sample under controlled temperature and atmosphere. This approach can access a wide temperature range (room temperature to 700℃) and maintain precise gas control (oxygen partial pressure down to 1-10 ppm without reactive gas) relevant to mixed ionic-electronic conducting oxides. We apply our technique to study three important materials with different configurations: Y-ZrO₂ (YSZ, 9.5 mol% Y₂O₃) single crystal, Gd-CeO₂ (GDC, 3% Gd) polycrystalline thin film, and Pr₀.₁Ce₀.₉O₂₋ₓ (PCO10) polycrystalline thin film. Using this experimental platform, we directly observe strain-induced oxygen breathing from the mixed ionic electronic conductor PCO10 and demonstrate the strain effect on ion transport and surface exchange efficiency. Tensile strain is found to consistently increase the conductivity in all three materials under all temperatures, with much more significant effect on PCO10. Finally, tensile strain does not show a significant effect on the surface oxygen exchange coefficients of PCO10 due to a cancellation from the simultaneous change of activation energy and preexponential factors.
This thesis quantifies the effect of external strain and electric field on defect chemistry in functional complex oxides. Our new methodology, developed framework, and deep insights into energy conversion/storage and memory materials can be widely applied to applications including batteries, ferroelectrics, semiconductors, and catalysis.
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