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

Abstract

In this work, we study the further atomic details of the extended mixed dislocations, in particular the local chemistry, in a SrTiO 3 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 ¼ a[201]/(100) extended mixed dislocation, which we named as local crystallographic shear (LCS) structures. In addition, we identify a rock salt FCC-type TiO x (x ¼ 0.66-1.24) phase at the locations of the extended mixed dislocations. In contrast to the insulating TiO 2 phases, the TiO x phase is known to exhibit very low electrical resistivity of only several mU cm. In this regard, the extended mixed dislocations of SrTiO 3 comprising the FCC TiO x phase may function as the conducting filament in resistive switching processes by completion and disruption of the TiO x phase along the dislocation cores through electrically stimulated redox reactions.

The Electronic Properties of Extended Defects in SrTiO3—A Case Study of a Real Bicrystal Boundary

“…According to TEM analysis, one can distinguish two types of dislocations. Type "A" has a core consisting of αSrO + βTiO2 + γTi2O3, which causes flexoelectric polarization, while type "B" has a structure of βTiO2 + γTi2O3 + δTiO, causing ferroelectric polarization [38][39][40]. When trying to understand the electric conductivity of SrTiO 3 , the impact of dislocations must be taken into account, not only at the boundary itself but also in the surface layer of the polished crystal.…”

“…According to TEM analysis, one can distinguish two types of dislocations. Type "A" has a core consisting of αSrO + βTiO 2 + γTi 2 O 3 , which causes flexoelectric polarization, while type "B" has a structure of βTiO 2 + γTi 2 O 3 + δTiO, causing ferroelectric polarization [38][39][40].…”

Is Reduced Strontium Titanate a Semiconductor or a Metal?

“…Hence, a heterogeneous surface layer is generated that is depleted in Sr but enriched in oxygen vacancies featuring a filamentary metallic conductivity. Upon high temperature reduction, even the crystallographic structure close to the dislocations themselves could change towards that of Ti suboxides, thus building up a metallic structure resistant to room temperature oxidation [37,38]. Upon high temperature oxidation, the opposite effects will occur.…”

Defect phases – thermodynamics and impact on material properties

“…In ordered crystals, this occurs naturally even in perfect stoichiometry. Intermetallics are the most closely related crystals to the metallic materials discussed elsewhere in this review, however, many other non-metallic material systems, such as transition metal carbides [143][144][145][146] and perovskites [147,148], show similarly ordered crystal structures and complex defects. Examples from the literature for dislocations, grain and phase boundaries are shown in Figure 15.…”