Dislocations in SrTiO3

The atomic structure of a SrTiO3 dislocation is revealed directly by phase-retrieval electron microscopy. In particular, atomic columns of light oxygen are observed simultaneously with the columns of considerably heavier Sr and Ti. A distinct structural modification of the oxygen octahedra at the dislocation core as well as a significant nonstoichiometry, including a deficiency of oxygen, are observed. Deviations from the bulk chemical concentration are quantified column by column by means of structure modeling and quantum-mechanical simulations of the electron scattering process.

A screw dislocation in SrTiO3 has been studied taking benefit of NCSI in a spherical aberration-corrected transmission electron microscope. The core structure of the dislocation was found to be atomically resolved with respect to both cation and oxygen columns by the inclusion of the central core area. The dislocation core has been found to be characterized by a helical distortion of the lattice planes, which leads, in particular, to azimuthally elongated image dots associated with individual atomic columns close to the core centre. The atomic coordination in the dislocation core was identified and a high density of atom vacancies is revealed for a Ti-O column at the core.
The dislocations created by mechanical polishing of SrTiO3 (100) single crystals were investigated by means of transmission electron microscopy techniques combined with scanning transmission electron microscopy techniques.

A high density of dislocations (higher than 1011 cm–2） was observed in the surface layer with a thickness of about 5 μm. These dislocations were found to be straight and highly aligned along the 〈111〉 directions. In most cases they appear in pairs or as a bundle. The nature of the dislocations was determined as mixed 〈110〉-type with the line vectort parallel to 〈111〉. They are 〈110〉-type 35.26º dislocations. The isolated 〈110〉-type 35.26º dislocations possess a compact core structure with a core spreading of ~0.5 nm. Dissociation of the dislocation occurs on the {1−10} glide plane, leading to the formation of two b = a/2〈110〉 partials separated by a stacking fault. The separation of the two partials was estimated to be 2.53±0.32 nm based on a cross-correlation analysis of atomic-resolution images. Our results provide a solid experimental evidence for this special type of dislocation in SrTiO3. The high density of straight and highly 〈111〉-orientated dislocations is expected to have an important influence on the anisotropy in electrical and mass transport properties.

For dislocation cores at a 6º low-angle [001] tilt grain boundary in SrTiO3, an embedded TiOx rocksalt-like structure has been suggested, consistent with a deficiency of Sr. However, direct evidence supporting these suggestions has not been obtained up to now. We reveal the atomic structure and chemistry of edge dislocation cores at a low-angle [001] symmetric tilt-boundary in SrTiO3 bicrystals by imaging techniques of high-angle annular dark-field scanning transmission electron microscopy and electron energy loss spectroscopy with an FEI Titan cube3 60-300 (PICO) microscope operated at 80 kV. The experimental results demonstrate direct evidence for a local coordination of edge-sharing TiO6 octahedra at the dislocation cores, which can be understood as the result of strain. The local coordination of edge-sharing TiO6 octahedra is associated with the face-centered cubic NaCl-type TiO phase. The present study therefore provides a solid structural and chemical basis for understanding the properties of dislocations.

Contact:

Dr. Chun-Lin Jia
Phone: +49 2461 61-2408
E-Mail: c.jia@fz-juelich.de