Perovskite materials can exhibit a wide range of properties like superconductivity, magnetism and piezoelectricity. These diverse functional properties emerge due to strong structure -function coupling. Small changes in lattice structure can lead to large changes in functional properties. In particular, perovskite heterostructures have recently attracted much interest due to the possibility of tailoring properties through coupling between substrate and film. However, due to the many different

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... rameters that can affect the properties of these materials, one needs detailed information about the structure of the films, especially at the film/substrate interface. In particular, the promotion or suppression of octahedral tilt ordering at interfaces, and the measurement of this using scanning transmission electron microscopy (STEM) is of great interest [1-3]. Ideally one wants to have precise information about every atom position to get the full 3-D structure. Using STEM one can normally get high resolution structural information about the axes orthogonal to the electron beam, however this still leaves one axis unaccounted for. Normally, two different TEM-lamellae thinned on two orthogonal zone axes is required to get this information about all three axes. However, extracting all this from one region of one sample would obviously be advantageous. Recently, advances in electron counting devices have enabled the development of fast 2-D pixelated STEM detectors. Such detectors can acquire a diffraction pattern for each probe position in a STEM scan, yielding a 4-D dataset: two spatial dimensions in the sample plane and two reciprocal dimensions in the detector plane. This allows for imaging of large parts of the convergent beam electron diffraction pattern (CBED) generated when doing STEM. This diffraction pattern contains a wealth of information, including the Higher Order Laue Zones (HOLZ), which reveals information about the crystallographic ordering parallel to the electron beam [4]. In this work, we have used a probe corrected ARM200cF equipped with a Medipix3 pixelated STEM detector to acquire CBED datasets across a La0.7Sr0.3MnO3/LaFeO3 (LSMO/LFO) heterostructure grown epitaxially on a SrTiO3-(111) substrate. By radially integrating these diffraction patterns, the datasets are analysed using a model-based approach similar to that previously used in EELS processing [5]. By fitting a Gaussian function to the integrated HOLZ circle (shown in Fig. 1c) , information about the lattice size parallel to the electron beam is extracted for both the films and substrate: the HOLZ ring chosen corresponds to a period doubling of the perovskite lattice along the beam direction. By correlating this data to standard atomically resolved STEM images, a more complete picture of the 3-D structure could be obtained. Results from the model-based data processing can be seen in Fig. 1(e-f) , which show the integrated intensity of the Gaussian fitted to the HOLZ ring. This shows clearly that this HOLZ ring increases gradually in intensity over several unit cells in the LFO before reaching a maximum, and then falling again in intensity towards and into the LSMO layer. The obvious reason for the appearance of a HOLZ ring corresponding to unit cell doubling in a perovskite would be a octahedral tilting transition, and it is known that bulk LFO has an orthorhombic structure with a doubled