Local crystallographic shear structures in a[201] extended mixed dislocations of SrTiO3 unraveled by atomic-scale imaging using transmission electron microscopy and spectroscopy.

Hongchu Du,Chun-Lin Jia,Joachim Mayer

doi:10.1039/c8fd00102b

Abstract

Recently, extended mixed dislocations were observed at a [001]/(100) low-angle tilt grain boundary of a SrTiO3 bicrystal because of a slight twist between the two crystal parts. The b = a[201]/(100) mixed dislocations at the grain boundary dissociate into three dislocations with Burgers vector b of a/2[101], a[100], and a/2[101], respectively. A structure model has been proposed in particular for the dislocation cores of the two partials with b = a/2[101] based on the high-angle annular dark-field (HAADF) images acquired by scanning transmission electron microscopy (STEM). However, the details of the atomic structure and the chemical composition of the dislocation cores remain unexplored, especially for the b = a[100] dislocation that is evidently disassociated into two b = a/2[101] partial dislocations. In this work, we study the further atomic details of the extended mixed dislocations, in particular the local chemistry, in a SrTiO3 bicrystal using STEM, electron energy loss spectroscopy (EELS), and energy dispersive X-ray (EDX) spectroscopy techniques. By these atomic-scale imaging techniques, we reveal a unique feature for the atomic structure of the b = a[201]/(100) extended mixed dislocation, which we named as local crystallographic shear (LCS) structures. In addition, we identify a rock salt FCC-type TiOx (x = 0.66-1.24) phase at the locations of the extended mixed dislocations. In contrast to the insulating TiO2 phases, the TiOx phase is known to exhibit very low electrical resistivity of only several μΩ cm. In this regard, the extended mixed dislocations of SrTiO3 comprising the FCC TiOx phase may function as the conducting filament in resistive switching processes by completion and disruption of the TiOx phase along the dislocation cores through electrically stimulated redox reactions.

Highlights

PaperPerovskite oxides have a general formula of ABO3, in which the smaller B cations are 6-fold coordinated with the oxygen (O) atoms forming BO6, thereby occupying the octahedral interstices, whereas the larger A cations occupy the space between the corner-sharing BO6 octahedra.1 The perovskite structure is adaptable to many different A-site and B-site cation species, and, allows for more than one type of cation species occupying the equivalent A-sites and B-sites
Similar to the a[100] edge dislocations found at low-angle tilt grain boundary, local TiO6 octahedra sharing edges is a unique feature for the atomic structure of the b 1⁄4 a[201]/(100) extended mixed dislocation, including the core structures of the b 1⁄4 a/2[101] partial dislocation, the b 1⁄4 a[100] complete dislocation, and the
Later on electron-paramagneticresonance measurements have revealed the existence of corner-shared TiO6– TiO6 octahedra in Ti-rich planar defects of oxygen-de cient rutile TiO2.47 In particular, with the application of aberration-corrected transmission electron microscopy (TEM) imaging techniques, an increasing number of local crystallographic shear (LCS) structures were observed at defects in different perovskite oxides including SrTiO3,24,39,48 BaTiO3,40 Ba-doped Na0.5Bi0.5TiO3,41 and (Nd,Ti)-codoped Bi0.9Nd0.15FeO3.42,43 These observations indicate that LCS-type defects appear to be common in B-rich ABO3 perovskite oxides

Summary

Introduction

PaperPerovskite oxides have a general formula of ABO3, in which the smaller B cations are 6-fold coordinated with the oxygen (O) atoms forming BO6, thereby occupying the octahedral interstices, whereas the larger A cations occupy the space between the corner-sharing BO6 octahedra. The perovskite structure is adaptable to many different A-site and B-site cation species, and, allows for more than one type of cation species occupying the equivalent A-sites and B-sites. The perovskite structure is adaptable to many different A-site and B-site cation species, and, allows for more than one type of cation species occupying the equivalent A-sites and B-sites. This leads to a large number and variety of perovskite-derived oxides with many well-known and emergent properties e.g. superconductivity, ferroelectricity, electronic/ionic conductivity, and 2D electron gas phenomena that are currently being studied for a wide range of technological applications. In order to systematically elucidate the microstructure–property relationships in perovskite oxides, dislocations at bicrystal grain boundaries have been extensively studied.. Strontium titanate (SrTiO3) is a representative perovskite oxide and is a well-known prototype material for resistive switching. At symmetrical tilt low angle grain boundaries of SrTiO3, edge dislocations are expected to be uniformly spaced. Besides edge dislocations, in reality faceting and extended dislocations may occur at the tilt grain boundaries and exert effects on the structural and electrical properties. In order to establish the microstructure–property relationships, it is a prerequisite to study comprehensively atomic structural details of different types of dislocations in real bicrystals

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Discussion

Conclusion