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Determining the 3-dimensional crystallography of a material withsub-nanometre resolution is essential to understanding strain effects inepitaxial thin films. A new scanning transmission electron microscopy imagingtechnique is demonstrated that visualises the presence and strength of atomicmovements leading to a period doubling of the unit cell along the beamdirection, using the intensity in an extra Laue zone ring in the back focalplane recorded using a pixelated detector method. This method is used togetherwith conventional atomic resolution imaging in the plane perpendicular to thebeam direction to gain information about the 3D crystal structure in anepitaxial thin film of LaFeO3 sandwiched between a substrate of (111) SrTiO3and a top layer of La0.7Sr0.3MnO3. It is found that a hitherto unreportedstructure of LaFeO3 is formed under the unusual combination of compressivestrain and (111) growth, which is triclinic with a periodicity doubling fromprimitive perovskite along one of the three <110> directions lying in thegrowth plane. This results from a combination of La-site modulation along thebeam direction, and modulation of oxygen positions resulting from octahedraltilting. This transition to the period-doubled cell is suppressed near both thesubstrate and near the La0.7Sr0.3MnO3 top layer due to the clamping of theoctahedral tilting by the absence of tilting in the substrate and due to anincompatible tilt pattern being present in the La0.7Sr0.3MnO3 layer. This workshows a rapid and easy way of scanning for such transitions in thin films orother systems where disorder-order transitions or domain structures may bepresent and does not require the use of atomic resolution imaging, and could bedone on any scanning TEM instrument equipped with a suitable camera.

3D sub-nanoscale imaging of unit cell doubling due to octahedral tilting and cation modulation in strained perovskite thin films