Abstract
Determining the 3-dimensional crystallography of a material with sub-nanometre resolution is essential to understanding strain effects in epitaxial thin films. A new scanning transmission electron microscopy imaging technique is demonstrated that visualises the presence and strength of atomic movements leading to a period doubling of the unit cell along the beam direction, using the intensity in an extra Laue zone ring in the back focal plane recorded using a pixelated detector method. This method is used together with conventional atomic resolution imaging in the plane perpendicular to the beam direction to gain information about the 3D crystal structure in an epitaxial thin film of LaFeO3 sandwiched between a substrate of (111) SrTiO3 and a top layer of La0.7Sr0.3MnO3. It is found that a hitherto unreported structure of LaFeO3 is formed under the unusual combination of compressive strain and (111) growth, which is triclinic with a periodicity doubling from primitive perovskite along one of the three <110> directions lying in the growth plane. This results from a combination of La-site modulation along the beam direction, and modulation of oxygen positions resulting from octahedral tilting. This transition to the period-doubled cell is suppressed near both the substrate and near the La0.7Sr0.3MnO3 top layer due to the clamping of the octahedral tilting by the absence of tilting in the substrate and due to an incompatible tilt pattern being present in the La0.7Sr0.3MnO3 layer. This work shows a rapid and easy way of scanning for such transitions in thin films or other systems where disorder-order transitions or domain structures may be present and does not require the use of atomic resolution imaging, and could be done on any scanning TEM instrument equipped with a suitable camera.
Highlights
In complex oxides functional properties are coupled to the crystal structure, and control of the crystal structure enables engineering of functional properties
Using a 4D scanning transmission electron microscope (STEM) approach we can map the periodicity and the magnitude of atomic movements along the beam direction by imaging the intensity of scattering into specific higher-order Laue zone rings. This adds information about the third dimension to STEM characterization of crystals, including cell doubling from octahedral tilting and associated cation movements, without needing to make multiple lamellae or use high tilt tomography
In combination with atomic resolution imaging of the same lamella, this allows a lot of detail about the local 3D “crystal” structure of a perovskite oxide thin-film system to be found from one orientation alone, in this case allowing an approximate atomic model to be developed that could be further refined using Density functional theory (DFT)
Summary
In complex oxides functional properties are coupled to the crystal structure, and control of the crystal structure enables engineering of functional properties. An ubiquitous feature of perovskites and related structures is tilting of the oxygen octahedra. While this has been well known for a long time, and the simple tilt patterns were classified many years ago in the seminal work of Glazer [1], it is still a lively subject for investigation. There has been a great development in tuning the tilting pattern using composition or strain to obtain emergent properties in thin films [2,3]. Of special interest has been the evolution to tilt patterns at epitaxial interfaces and subsequent emergence of novel functional properties. The interface between antiferromagnetic (AF) X FeO3 (X = La, Bi) and the ferromagnetic
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