Ultrafast collective oxygen-vacancy flow in Ca-doped BiFeO3

Ji Soo Lim,Ramamoorthy Ramesh,Tae Yeong Koo,Lane W Martin,Jin Hong Lee,Chan-Ho Yang,Ran Gao,Heung-Sik Park

doi:10.1038/s41427-018-0087-5

Abstract

The ultrafast motion of oxygen vacancies in solids is crucial for various future applications, such as oxide electrolytes. Visualization and quantification can offer unforeseen opportunities to probe the collective dynamics of defects in crystalline solids, but little research has been conducted on oxygen vacancy electromigration using these approaches. Here, we visualize electric-field-induced creation and propagation of oxygen-vacancy-rich and -poor competing phases and their interface with optical contrast in Ca-substituted BiFeO3 that contains a high density of mobile oxygen vacancies. We quantitatively determined the drift velocity of collective migration to be on the order of 100 μm s−1 with an activation barrier of 0.79 eV, indicating a significantly large ionic mobility of 2 × 10−6 cm2 s−1 V−1 at a remarkably low temperature of 390 °C. In addition, visualization enables direct observation of fluidic behavior, such as the enhancement of conduction at channel edges, which results in U-shaped viscous propagation of the phase boundary and turbulence under a reverse electric field. All of these results provide new insights into the collective motion of defects.

Highlights

Ionized oxygen vacancies act as doubly charged electron donors in oxides and affect a variety of physical properties, such as superconductivity[1], magnetoresistance[2], and ferroelectricity[3,4]
A vast amount of research interest has been devoted to technological applications, including resistive switching memory[5], solid oxide fuel cells (SOFCs)[6], batteries[7], sensors[8], memresistors[9], and neuromorphic computation[10], that use the mobility of these species
By tracking the visualized electroforming processes, we were able to directly observe the propagation of the boundary between the oxygenvacancy-rich and -poor competing phases through a channel protected by an oxygen barrier and quantitatively determine the kinematic and thermodynamic variables of the ionic motion

Summary

Introduction

Ionized oxygen vacancies act as doubly charged electron donors in oxides and affect a variety of physical properties, such as superconductivity[1], magnetoresistance[2], and ferroelectricity[3,4]. Ionic conduction relies on ions hopping through an otherwise rigid crystal, and multiple vacancies likely interact with each other at the high-density limit of the defect concentration, studies on the dynamics of ionic conductivity have primarily focused on single particles[11]. Structural, electronic, and optical states are strongly influenced by the defect concentration. Defect formation and migration in solids is a highly complex mutual coupling phenomenon that influences effects ranging from structural and electronic phase transitions to diffusional processes under inhomogeneous external/internal electric and strain fields and chemical pressures[13,14,15,16]. To pave the way for understanding this sophisticated dynamic phenomenon, it is necessary to better visualize the spatial distribution of the oxygen-vacancy concentration and observe the real-time evolution of the distribution in electric fields

Methods

Findings

Conclusion