  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.
