Local crystallographic shear structures in a [201] extended mixed dislocations of SrTiO 3 unraveled by atomic-scale imaging using transmission electron microscopy and spectroscopy

Recently, extended mixed dislocations were observed at a [001]/(100) low-angle tilt grain boundary of a SrTiO 3 bicrystal because of a slight twist between the two crystal parts. The b ¼ 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 ¼ a /2[101] based on the high-angle annular dark- ﬁ eld (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 ¼ a [100] dislocation that is evidently disassociated into two b ¼ 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 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 ﬁ lament in resistive switching processes by completion and disruption of the TiO x phase along the dislocation cores through electrically stimulated redox reactions. HAADF-STEM and imaging to the atomic structure details of the extended mixed [201]/(100) dislocations at 3 (cid:2) [001]/(100) tilt grain boundaries in SrTiO 3 bicrystals. Our results reveal segregation of an FCC TiO x phase along the middle complete dislocation core with b ¼ a [100]. The FCC TiO x phase is connected to the two perovskite structural parts of the bicrystal by edge-sharing TiO 6 blocks, which we refer to as local crystallographic shear (LCS) structures. More interestingly, the partial dislocation ( b ¼ a /2 [101]) cores, as well as the stacking fault through which they connect to the middle complete dislocation, also consist of LCS structures as the consequence of the screw component from the slight twist between the two crystal parts. The stacking faults are revealed to have a more complicated scenario than that of two neighboring {001} Ti – O layers.

structure model has been proposed in particular for the dislocation cores of the two partials with b ¼ 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 ¼ a[100] dislocation that is evidently disassociated into two b ¼ 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 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. a Introduction Perovskite oxides have a general formula of ABO 3 , in which the smaller B cations are 6-fold coordinated with the oxygen (O) atoms forming BO 6 , thereby occupying the octahedral interstices, whereas the larger A cations occupy the space between the corner-sharing BO 6 octahedra. 1 The perovskite structure is adaptable to many different A-site and B-site cation species, and, meanwhile, 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, 2 ferroelectricity, 3 electronic/ionic conductivity, 4 and 2D electron gas phenomena 5 that are currently being studied for a wide range of technological applications.
Recent studies have shown that dislocations in perovskite oxides have decisive effects on their functional properties. [6][7][8][9][10] In order to systematically elucidate the microstructure-property relationships in perovskite oxides, dislocations at bicrystal grain boundaries have been extensively studied. [11][12][13] This is mainly because, as model systems, the type and the density of dislocations at bicrystal grain boundaries can be controlled to a large extent by adjusting the misorientation between the two crystal parts. Strontium titanate (SrTiO 3 ) is a representative perovskite oxide and is a well-known prototype material for resistive switching. 14 At symmetrical tilt low angle grain boundaries of SrTiO 3 , edge dislocations are expected to be uniformly spaced. 15 Besides edge dislocations, in reality faceting and extended dislocations may also occur at the tilt grain boundaries and exert effects on the structural and electrical properties. 16 In order to establish the microstructure-property relationships, it is therefore a prerequisite to study comprehensively atomic structural details of different types of dislocations in real bicrystals.
With the advent of spherical aberration (C S )-corrected transmission electron microscopy (TEM), it is now becoming possible to routinely obtain structural, chemical, and bonding information of defects at genuine atomic resolution from experimental observations. Using high-angle annular dark-eld scanning transmission electron microcopy (HAADF-STEM) and spectrum imaging techniques, direct experimental evidence for the face-centered cubic (FCC) NaCl-type titanium monoxide (TiO x ) phase at the dislocation cores in low-angle tilt grain boundaries of SrTiO 3 has been presented in previous studies. 15 18 Nevertheless, details on the atomic structure of the extended mixed dislocation core remain unclear.
Because at defects atomic intermixing, partial occupancy, and atomic random displacement oen happen, the interpretation of the HAADF images of defects is complicated and hence needs caution. In this work, we use a combination of HAADF-STEM and EDX/EELS spectrum imaging techniques to clarify the atomic structure details of the extended mixed [201]/(100) dislocations at 3 low-angle [001]/(100) tilt grain boundaries in SrTiO 3 bicrystals. Our results reveal segregation of an FCC TiO x phase along the middle complete dislocation core with b ¼ a [100]. The FCC TiO x phase is connected to the two perovskite structural parts of the bicrystal by edge-sharing TiO 6 blocks, which we refer to as local crystallographic shear (LCS) structures. More interestingly, the partial dislocation (b ¼ a/2 [101]) cores, as well as the stacking fault through which they connect to the middle complete dislocation, also consist of LCS structures as the consequence of the screw component from the slight twist between the two crystal parts. The stacking faults are revealed to have a more complicated scenario than that of two neighboring {001} Ti-O layers.

Experimental
A 3 h001i/{100} tilt-bicrystal of SrTiO 3 was obtained commercially from CrysTec GmbH. TEM lamellae were prepared by focused ion beam (FIB) milling with an FEI Helios NanoLab 400S, 19 further thinned by a Bal-Tec Res-120 system, and nally cleaned by a Fischione Nanomill 1040 system. The HAADF-STEM and EDX spectrum imaging was conducted at 200 kV with an FEI Titan G2 80-200 Chem-iSTEM microscope with an 18 mrad probe semi-convergent angle and a 67 mrad inner collection angle of the HAADF detector. 20 The EELS spectrum imaging was carried out at 80 kV with an FEI Titan 3 60-300 (PICO) microscope with a 15 mrad probe semi-convergent angle and a 53 mrad collection angle. 21 HAADF-STEM image simulations were carried out using the Dr. Probe soware package. 22 The following simulation parameters were used in this work regardless of structural models: acceleration voltage of 200 kV, specimen thickness of 32 nm, semi-convergent angle of 18 mrad, collection angle of 67-175 mrad, and pixel size of 0.01 nm. The structural models were drawn using VESTA soware. 23 Rigid iterative image registration and frame averaging were applied for reducing the noise of the atomic resolution HAADF-STEM images. 24 A nonlinear ltering algorithm was employed for smoothing the EDX maps. 25 Multivariate data analyses were performed on the EELS spectrum data to determine the constituent spectra and their distribution. 26

Results
Atomic scale imaging of the extended mixed dislocation In the atomic resolution HAADF-STEM image, the intensity is approximately in proportion to Z 2 , where Z is the atomic number, whereby the brighter and less bright contrast corresponds to the Sr and Ti-O columns, respectively. However, caution should be exercised at the dislocation cores, where intermixing of different types of atoms, partial occupancy of atoms, as well as deviations of atoms from a line along the projection direction may occur and thus considerably inuence the intensity of the HAADF-STEM image. Hence, we used the atomic-scale EDX spectrum imaging technique to investigate the chemistry of the atomic columns at the core areas. The atomic resolution EDX maps for Sr and Ti elements are shown also in Fig. 2, which provide direct evidence for Ti enrichment at the dislocation core of the middle b ¼ a[100] complete dislocation. The observed atomic structure of the dislocation cores is rather more complicated than the structural model proposed in the previous report. 18 Core-loss electron EELS probes the excitation of electrons from deeply bound initial core states to nal unoccupied electronic states above the Fermi level by electric dipole transitions. The initial states are characteristic of the specic type of atom, whereas the nal states correspond to the unoccupied partial density of states of the selected atom that is allowed by the electric dipole selection rule with the presence of a core hole. Core-loss EELS spectra therefore provide local chemistry, bonding and electronic structure including both the atomic and electronic arrangements. 27 In the present work, we applied core-loss EELS to  characterize the Ti-O bonding at the dislocation cores and the stacking faults. Fig. 3 shows the EELS spectra of Ti L 2,3 and O K edges. The Ti L 2,3 spectrum from the middle b ¼ a[100] complete dislocation core is very similar to that of TiO phase with a NaCl type structure, 28 for which the crystal eld splitting of the L 2 and L 3 peaks almost completely vanishes. Correspondingly, the ne structure details for the O K EELS spectrum from the dislocation core signicantly diminish in comparison with those from the normal SrTiO 3 lattice away from the dislocation core, which is further consistent with those of the TiO phase. 28 Fig. 4 shows the HAADF-STEM image and EDX    5 shows the core-loss EELS spectra of Ti L 2,3 and O K edges obtained at the lower dislocation core shown in Fig. 4. The Ti L 2,3 core-loss spectrum from the dislocation core has less crystal eld splitting of the L 2 and L 3 peaks evident than the normal Ti 4+ spectrum from locations away from the dislocation core. But the crystal eld splitting of the L 2 and L 3 peaks from this normal b ¼ a[100] edge dislocation is still visible, being different from that of the middle b ¼ a[100] complete dislocation core in the extended mixed dislocation, for which the splitting almost completely disappears. The peak corresponding to the hybridization of O 2p and Sr 4d states in the O K edge decreases for the spectrum from the dislocation core compared with that from the normal SrTiO 3 around the dislocation core. These observations indicate a lower valence state than normal 4+ of the Ti atoms at the Ti-rich dislocation core.

Atomic structural model and multislice image simulation
Based on the experimental results from the atomic-scale imaging by HAADF-STEM, core-loss EELS, and EDX spectroscopy, atomic structural models for the b ¼ a/2[101] partial dislocation core and for the b ¼ a[100] complete dislocation core with segregation of a TiO phase were constructed. Fig. 6a shows the structural model for the core of the b ¼ a/2[101] partial dislocation, projected along the [001] direction. A close view of the octahedral conguration of TiO 6 and the atomic arrangement of the stacking fault projected along the [010] direction is shown in Fig. 6b and c, respectively. In the structure model the Sr and Ti atomic positions in the a-b plane were determined by tting the column peaks of the experimental HAADF-STEM image with a 2D Gaussian function. 30 The constructed atomic structural model for the b ¼ a/2[101] partial dislocation core is more complicated than a simple stacking fault scenario formed by missing a {001} Sr-O layer that has been previously proposed. 18 As shown in Fig. 6a, at the dislocation core a Ti-O column is missed (red dot circle). Two additional Ti-O columns (red shaded TiO 6 ) appear in which each octahedral TiO 6 shares edges with three adjacent TiO 6 to form an edge-sharing TiO 6 structure (Fig. 6b). The stacking fault, through which the b ¼ a/2[101] mixed partial dislocation core is extended to the b ¼ a[100] complete dislocation core, appears to be reconstructed rather than two neighboring {100} Ti-O lattice planes. As shown in Fig. 6c, the respective blue and yellow TiO 6 octahedra are assigned to the le and right SrTiO 3 lattices. As a consequence of the inter-faceting, the stacking fault has a nite region of 0.5 unit cells in the a-axis direction and comprises alternate edgesharing TiO 6 octahedral blocks and Sr atom pairs when viewing along the caxis. The atomic structure of the reconstructed stacking fault can therefore be described as a combination of local crystallographic shear and faceting on a unitcell scale.
Based on the atomic structural model, multislice image simulations were performed. Fig. 7 shows the close comparison between the experimental and the multislice simulated HAADF-STEM images of the b ¼ a/2[101] partial dislocation. A good match between the experimental and simulated images is achieved by taking the partial occupancy of Sr and Ti atoms into account, which is indicated by the maximum of their cross correlation of more than 0.92. Fig. 8a shows a magnied HAADF-STEM image with false color of the b ¼ a [100] complete dislocation core. In the image, the dots with a brighter contrast  (red-yellow) indicate the heavy Sr columns, whereas dots with a less bright contrast (blue-pink) signify the light Ti-O columns. The segregation of TiO phase at the dislocation core is clearly manifested by a distinct decrease of column intensity and the appearance of extra columns with respect to the HAADF image of the normal SrTiO 3 lattice. As shown in Fig. 8b, a TiO phase with width of 1.5 u.c. can be naturally incorporated in the dislocation core, i.e. between two SrTiO 3 crystal parts, which have a relative displacement of a/2[001] and are terminated with Ti-O (100) planes, thereby forming local crystallographic shear structures. Multislice image simulations were performed from the marked region of a SrTiO 3 /TiO/SrTiO 3 structural model in Fig. 8b both with (Fig. 8a right panel,  lower) and without (Fig. 8a right panel, upper) consideration of 25% of vacancies for both Ti and O atoms in the TiO block. The reason for the consideration of both Ti and O vacancies is because the FCC-type TiO x has a wide chemical composition range (x ¼ 0.66-1.24) of structural homogeneity, 29 and contains a high equilibrium concentration of vacancies in both the Ti-and O-sublattices, 31,32 even for stoichiometric TiO. 33 As seen from the right panel of Fig. 8a, the simulated image with consideration of both Ti and O vacancies shows a better match with the experimental image than that without consideration of atomic vacancies. Therefore, in addition to the direct evidence from EELS and EDX results, the multislice image simulation further supports the formation of a TiO x phase at the b ¼ a[100] complete dislocation core (Fig. 8b).

Discussion
Similar to the a[100] edge dislocations found at low-angle tilt grain boundary, local TiO 6 octahedra sharing edges is a unique feature for the atomic structure of the b ¼ a[201]/(100) extended mixed dislocation, including the core structures of the b ¼ a/2[101] partial dislocation, the b ¼ a[100] complete dislocation, and the stacking faults. In perovskite-related materials, structures comprising edge-or face-sharing MO 6 octahedra instead of corner-sharing in their parent structures have been known for a long time as crystallographic shear (CS) structures. 34 This provides the rationale for us to name local atomic structures consisting of metaloxygen (M-O) octahedra sharing edges in perovskite materials as local crystallographic shear (LCS) structures. Defects in perovskite oxides comprising LCS structures therefore can be classied as LCS-type defects. CS structures commonly occur in anion-decient oxides, e.g., W n O 3nÀ2 , (Mo,W) n O 3nÀ2 , Ti n O 2nÀ1 , etc., derived from the ReO 3 or rutile (TiO 2 ) structures based on metal-centered MO 6 coordination octahedra, 34 but rarely occur in the perovskite structure on the macro-scale. 34 42,43 These observations indicate that LCS-type defects appear to be common in B-rich ABO 3 perovskite oxides. Nevertheless, these observed local structures consisting of edge-sharing MO 6 octahedra have not been correlated with the well-known CS structures, and hence LCS-type defects have received much less attention than the well-known Ruddlesden-Popper-type defects in perovskite oxides. 44 More studies are therefore needed in order to understand how LCS-type defects alter local material properties.
Another prominent feature for the middle b ¼ a[100] complete dislocation in the b ¼ a[201]/(100) extended mixed dislocation is nanosized TiO phase segregated at the dislocation core along the dislocation line. The formation of the TiO x phase can be understood as a result of tensional and shear strain from the tilt and twist between the two crystal parts. Because of the larger lattice constant of the FCC TiO phase (a ¼ 0.42 nm) than SrTiO 3 (0.39 nm), forming the TiO x phase can accommodate tensional strain, and thus lowers the system energy. 15 On the other hand, a TiO x phase can be naturally incorporated between two SrTiO 3 crystal parts with a relative displacement of a/2[001] through forming local crystallographic shear structures (Fig. 8c), thereby accommodating the shear strain from the small twist.
The TiO x derived from the FCC-type structure is of particular interest because it tolerates a wide chemical composition range (x ¼ 0.66-1.24) of structural homogeneity, 29 and contains a high equilibrium concentration of vacancies in both the Ti-and O-sublattices, 31,32 even for stoichiometric TiO. 33 Moreover, these vacancies can be distributed either randomly or in an ordered way at the sites of the sublattices. 45 Furthermore, in contrast to TiO 2 phases being insulators or wide-band gap semiconductors, TiO x exhibits either semiconducting or metallic conductivity with electrical resistivity of only several mU cm. 46 From this perspective, dislocations comprising TiO x phase at their cores are of particular interest for study of resistive switching phenomena via changing the oxygen content of the TiO x phase through electrically stimulated redox reactions.
Locally conducting atomic force microscopy measurements have revealed that conducting laments can be preferentially created along the SrTiO 3 bicrystal boundary due to the easy reduction of the dislocation cores. 10 On the other hand, the electric conductivity for the cubic titanium monoxides TiO x varies with the oxygen content x. 46 A conceivable conguration of a resistive switching device making use of the extended mixed dislocation core is illustrated in Fig. 9. The oxygen content of the TiO x phase could be adjusted by electric-eld-induced redox reactions, thereby, setting different resistance states along the dislocation core. Please note that Fig. 9 illustrates a situation where the rate of oxygen release (incorporation) at the electrode is faster (slower) than that of the oxygen migration inside the material. In an opposite case, the variation of oxygen content should be reversed. 26

Conclusion
In conclusion, the atomic details of the extended mixed [100] complete dislocation. A survey of literature indicates that LCS-type defects appear to be common in B-rich ABO 3 perovskite oxides, which should deserve particular attention in order to understand their impact on local material properties. Moreover, we have observed an evident segregation of one-dimensional-like FCC TiO x phase at the middle b ¼ a[100] complete dislocation core. The formation of the TiO x phase can be understood as a result of tensional and shear strain from the tilt and twist between the two crystal parts. The dislocation core with a segregation of one-dimensional-like FCC TiO x phase appears to be a particularly interesting system for resistive switching, via a conceivable mechanism of completion and disruption of the TiO x phase through electrically stimulated redox reactions. Fig. 9 Schematic of a conceivable resistive switching configuration making use of the extended mixed dislocation core comprising a TiO x phase, under a situation where the rate of oxygen release (incorporation) at the electrode is faster (slower) than that of the migration of oxygen inside the lattice of the material.