We present a paradigm for fatigue in ferroelectric perovskite oxides: That of a structural phase transition in which oxygen vacancies order into two-dimensional planar arrays capable of pinning domain wall motion.

Oxygen-vacancy ordering as a fatigue mechanism in perovskite ferroelectrics

An analytic expression in closed form is given for “fatigue,” the dependence of P(N), the switched charge per unit area P versus switching event number N, in ABO3 perovskite structure ferroelectric thin films is given. The analysis is based upon Arlt’s model for fatigue in bulk perovskites, which involves preferential electromigration of oxygen vacancies to sites parallel to the electrode–ferroelectric interface plane, together with some arguments by Brennan on the effectiveness of such defect planes in pinning domain walls. The model is applied to PZT/Pt with no adjustable parameters and yields in complete agreement with experimental data the dependence of P(N) at different frequencies, different voltages, and different temperatures. Notably, unlike other proposals in the literature, the model does not involve any charge injection from the electrodes.

A model for fatigue in ferroelectric perovskite thin films

The kinetics of stoichiometry change of an oxide--a prototype of a simple solid-state reaction and a process of substantial technological relevance--is studied and analyzed in great detail. Oxygen incorporation into strontium titanate was chosen as a model process. The complete reaction can be phenomenologically and mechanistically understood beginning with the surface reaction and ending with the transport in the perovskite. Key elements are a detailed knowledge of the defect chemistry of the perovskite as well as the application of a variety of experimental and theoretical tools, many of them evolving from this study. The importance of the reaction and transport steps for (electro)chemical applications is emphasized.

How is oxygen incorporated into oxides? A comprehensive kinetic study of a simple solid-state reaction with SrTiO3 as a model material.

The electronic structure, defect chemistry, and transport properties of members of the mixed ionic electronic conducting SrTi1-xFexO3-y (STF) solid-solution system are revisited, and an improved defect chemical model is proposed in which Fe is considered to be one of the main constituents that shape the energy-band structure of STF, rather than an impurity dopant with acceptor-like character. As a consequence of the high inherent deficiency in the oxygen sublattice, introduced by the mixed-valence states of the B-site cations Ti4+ and Fe3+, oxygen vacancies and interstitials generated by the anion Frenkel reaction dominate the defect equilibria, leading to predominant ionic conductivity at intermediate partial pressures of oxygen. Increasing Fe content results in both a systematic decrease in band-gap energy, Eg0 = 3.2 − 1.9x + 0.5x2 eV, and reduction enthalpy, ΔHred = 5.8 − 3.4x + 1.7x2 eV. The decrease in band gap is explained on the basis of the systematic broadening of the Fe-derived 3d band lying abo...

Electronic Structure, Defect Chemistry, and Transport Properties of SrTi1-xFexO3-y Solid Solutions

We review oxygen K-edge X-ray absorption spectra of both molecules and solids. We start with an overview of the main experimental aspects of oxygen K-edge X-ray absorption measurements including X-ray sources, monochromators, and detection schemes. Many recent oxygen K-edge studies combine X-ray absorption with time and spatially resolved measurements and/or operando conditions. The main theoretical and conceptual approximations for the simulation of oxygen K-edges are discussed in the Theory section. We subsequently discuss oxygen atoms and ions, binary molecules, water, and larger molecules containing oxygen, including biomolecular systems. The largest part of the review deals with the experimental results for solid oxides, starting from s- and p-electron oxides. Examples of theoretical simulations for these oxides are introduced in order to show how accurate a DFT description can be in the case of s and p electron overlap. We discuss the general analysis of the 3d transition metal oxides including discussions of the crystal field effect and the effects and trends in oxidation state and covalency. In addition to the general concepts, we give a systematic overview of the oxygen K-edges element by element, for the s-, p-, d-, and f-electron systems.

Renato Bugge

Papers

THE DEFECT STRUCTUFE OF SrTi1−xFexO3−y (x = 0–0.8) INVESTIGATED BY ELECTRICAL CONDUCTIVITY MEASUREMENTS AND ELECTRON ENERGY LOSS SPECTROSCOPY (EELS)