SCIENTIFIC HIGHLIGHTS

Cross talks between strain and chemistry at dislocations

New insight into the nanochemistry and electronic structure of an oxide dislocation core: oxygen vacancies singled out and quantified

Dislocations are topological defects ubiquitous in crystals, which despite having been conceptually conceived more than eight decades ago, have largely remained in the drawers of oddities in materials science labs owing to the obstinate inaccessibility of their ~ 1nm cores. This is particularly true for dislocations in oxides, where electrostatic interactions arising from their ionic character can define the core structure and the defect chemistry in the associated strain field.

The strong sensitivity of the defect chemistry, particularly oxygen vacancy formation energies, to dislocation strains, otherwise provide a rich scenario for the development of new confined states. In this sense, dislocations are emerging as the one-dimensional analogue of ferroelastic or ferroelectric domain walls, where strain and symmetry breaking promote the development of localized states exhibiting different properties from those of the host crystal.

The transformation of such defects from passive into potentially active functional elements, however, necessitates a deep understanding of their chemical and electronic structure. In [1], we combine different atomic resolution imaging and spectroscopic techniques in the transmission electron microscope to determine the complex structure of misfit dislocations in the perovskite type La0.67Sr0.33MnO3/LaAlO3 heteroepitaxial system. While the position of the film–substrate interface is blurred by cation intermixing, oxygen vacancies selectively accumulate at the tensile region of the dislocation strain field. Such accumulation of vacancies is accompanied by the reduction of manganese cations in the same area, with associated chemical expansion effects contributing to accommodate the dislocation strain. The formation of oxygen vacancies is only partially electrically compensated and results in a positive net charge q ≈ +0.3 ± 0.1 localized in the tensile region of the dislocation, while the compressive region remains neutral. These results highlight a prototypical core model for perovskite-based heteroepitaxial systems and offer insights for a predictive manipulation of misfit dislocation properties.

Authors:
Núria Bagués,1,2 José Santiso,2 Bryan D. Esser,3 Robert E. A. Williams,3 Dave W. McComb,3 Zorica Konstantinovic,4 Lluís Balcells,1 Felip Sandiumenge1

Affiliations:
1Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Spain.
2Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Spain.
3Center for Electron Microscopy and Analysis. The Ohio State University, USA.
4Center for Solid State Physics and New Materials, Institute of Physics Belgrade, University of Belgrade, Serbia.

Publication:
The Misfit Dislocation Core Phase in Complex Oxide Heteroepitaxy
Adv. Funct. Mater. 28, 170443 (2018)
DOI: 10.1002/adfm.201704437

Figure:
(a) Atomic resolution high angle annular dark field image of misfit dislocation. The two arrows indicate the position of the two extra half planes of the dissociated core. These planes are hardly visible in the HAADF image due to their lower atomic number. Dotted lines draw a Burgers circuit, with the yellow arrow signifying the Burgers vector b=a[100]. 
(b) Atomic model of the dislocation core. 
(c) Schematic illustration of the basic mechanisms operating in the MD core. Red and blue represent tensile and compressive regions, respectively. The redox reaction indicated in the tensile region is displaced to the right, favoring the formation of electron donor oxygen vacancies. Each vacancy nominally releases two electrons which can reduce two neighboring Mn4+ cations. The imbalance between the rate of Sr diffusion out of the core region and the concentration of vacancies  results in a positive charge in the tensile region. The glide plane acts as a barrier for the redistribution of vacancies as indicated by crossed-out pathways. La/Mn antisite defects also form at the axial plane, on the tensile region, to accommodate the tensile strain.

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