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Lattice defects such as oxygen vacancies, interstitials, and their complexes are present in crystalline oxide materials. In particular, anti‐Frenkel defects, which refer to charge‐neutral anion vacancy‐interstitial pairs, are strongly coupled with ferroelectric and dielectric properties as electric dipoles. However, in order to observe their macroscopic manifestation, delicate defect controls are required to the extent that electronic and ionic charges are almost completely suppressed. Here, the thermal cycle dependence of dielectric and piezoelectric properties is scrutinized in the strain‐driven morphotropic phase boundaries of multiferroic La‐substituted BiFeO3 thin films. Electrochemical impedance spectroscopy provides the Warburg feature that is considered evidence of the ionic origin. The observations are discussed based on anti‐Frenkel defects that are created or annihilated reversibly by thermal cycles through high‐temperature structural phase transition temperature or magnetic Néel temperature. The defect dipoles are spontaneously aligned by the flexoelectric effect in the phase boundaries inducing a metastable interfacial ferroelectric phase. The findings offer useful insight into defect dipoles. [ABSTRACT FROM AUTHOR]

Oxygen vacancies are ubiquitous in oxides, and taking advantage of their mobility is the cornerstone for a variety of future applications. The visualization and quantification of collective defect flow based on electrochromism is a powerful approach to explore oxygen kinetics and electrochemical reaction even in cases that electronic conduction is considerably mixed, but whether or not the measured kinetic properties harmonize with those obtained by the conventional impedance spectroscopy remains veiled. Here, we identify complementary relationships between the two methods by investigating the oxygen vacancy transport in Ca 30%-doped bismuth ferrite thin films epitaxially grown on SrTiO3 (110) substrates. We find that the activation energy of ionic hopping is 0.78 (or 0.92 eV) for the application of an electric bias along [001] (or [1 1 ¯ 0]) due to the grain elongation along [001]. We anneal the films in an N2 gas environment at high temperatures to suppress the electronic contribution for access to standard impedance spectroscopy. The oxygen kinetic properties obtained from the two methods are consistent with each other, complementarily revealing the collective phase evolution as well as the ionic impedance of the bulk, grain boundary, and interfacial regions. These comparative works provide useful insights into ionic defect conduction in oxides in an intuitive and quantitative manner. [ABSTRACT FROM AUTHOR]

Ionic migration is a key ingredient for applications such as oxide electrolytes and resistive switching memories. We investigate the evolution of ionic conduction pathways based on optical contrast in an epitaxial Bi0.7Ca0.3FeO3−δ thin film where oxygen vacancies are spontaneously produced. We visualize electroforming processes in the hundred-micrometer-scale material channels between coplanar electrodes with a constant electric bias at an elevated temperature, systematically varying the channel orientation with respect to the crystal axis. At the initial stage of electroforming, conducting filaments are created and propagate nearly along the crystal axes ⟨100⟩. The local density of conducting filament regions increases with the elapsed time of bias application and also exhibits a linear dependence on the spatial position at a given time. We also find that the filament-type ionic conduction is abruptly transformed to the bulk conduction when the filament density reaches ∼30%. These results offer useful insight into collective ionic migration in crystalline solids. [ABSTRACT FROM AUTHOR]

Ionic migration is a key ingredient for applications such as oxide electrolytes and resistive switching memories. We investigate the evolution of ionic conduction pathways based on optical contrast in an epitaxial Bi0.7Ca0.3FeO3−δ thin film where oxygen vacancies are spontaneously produced. We visualize electroforming processes in the hundred-micrometer-scale material channels between coplanar electrodes with a constant electric bias at an elevated temperature, systematically varying the channel orientation with respect to the crystal axis. At the initial stage of electroforming, conducting filaments are created and propagate nearly along the crystal axes ⟨100⟩. The local density of conducting filament regions increases with the elapsed time of bias application and also exhibits a linear dependence on the spatial position at a given time. We also find that the filament-type ionic conduction is abruptly transformed to the bulk conduction when the filament density reaches ∼30%. These results offer useful insight into collective ionic migration in crystalline solids. [ABSTRACT FROM AUTHOR]

Ionic movement has received renewed attention in recent years, particularly in the field of ferroelectric oxides, since it is intrinsically linked to chemical reaction kinetics and ferroelectric phase stability. The associated surface electrochemical processes coupled local ionic transport with an applied electric bias, exhibiting very high ionic mobility at room temperature based on a simple electrostatics scenario. However, few studies have focused on the applied-polarity dependence of ionic migration with directly visualized maps. Here, we use incorporated experiments of conductive scanning probe microscopy and time-of-flight secondary ion mass spectrometry to investigate oxygen ionic migration and cation redistribution in ionic oxides. The local concentrations of oxygen vacancies and other cation species are visualized by three-dimensional mappings, indicating that oxygen vacancies tend to be ejected toward the surface. An accumulation of oxygen vacancies and ionic redistribution strongly depend on tip polarity, thus corroborating their role in the electrochemical process. This work illustrates the interplay between ionic kinetics and electric switching.

Mobile oxygen vacancies offer a substantial potential to broaden the range of optical functionalities of complex transition metal oxides due to their high mobility and the interplay with correlated electrons. Here, we report a large electro-absorptive optical variation induced by a topotactic transition via oxygen vacancy fluidic motion in calcium ferrite with large-scale uniformity. The coloration efficiency reaches ~80 cm 2 C -1 , which means that a 300-nm-thick layer blocks 99% of transmitted visible light by the electrical switching. By tracking the color propagation, oxygen vacancy mobility can be estimated to be 10 -8 cm 2 s -1 V -1 near 300°C, which is a giant value attained due to the mosaic pseudomonoclinic film stabilized on LaAlO 3 substrate. First-principles calculations reveal that the defect density modulation associated with hole charge injection causes a prominent change in electron correlation, resulting in the light absorption modulation. Our findings will pave the pathway for practical topotactic electrochromic applications., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).)