research papers J. Appl. Cryst. (2019). 52 https://doi.org/10.1107/S1600576719005569 1 of 11 Received 1 February 2019 Accepted 23 April 2019 Edited by V. Holy ´, Charles University, Prague, Czech Republic and CEITEC at Masaryk University, Brno, Czech Republic Keywords: electron diffraction; thin films; twinning; perovskites. Precession electron diffraction tomography on twinned crystals: application to CaTiO 3 thin films Gwladys Steciuk, a,b Adrian David, a Va ´clav Petr ˇı ´c ˇek, b Luka ´s ˇ Palatinus, b Bernard Mercey, a Wilfrid Prellier, a Alain Pautrat a and Philippe Boullay a * a CRISMAT, Normandie Universite ´, ENSICAEN, UNICAEN, CNRS UMR 6508, 6 Boulevard du Mare ´chal Juin, Caen, F-14050, France, and b Department of Structure Analysis, Institute of Physics, Czech Academy of Sciences, Na Slovance 2, Prague, Czech Republic. *Correspondence e-mail: philippe.boullay@ensicaen.fr Strain engineering via epitaxial thin-film synthesis is an efficient way to modify the crystal structure of a material in order to induce new features or improve existing properties. One of the challenges in this approach is to quantify structural changes occurring in these films. While X-ray diffraction is the most widely used technique for obtaining accurate structural information from bulk materials, severe limitations appear in the case of epitaxial thin films. This past decade, precession electron diffraction tomography has emerged as a relevant technique for the structural characterization of nano-sized materials. While its usefulness has already been demonstrated for solving the unknown structure of materials deposited in the form of thin films, the frequent existence of orientation variants within the film introduces a severe bias in the structure refinement, even when using the dynamical diffraction theory to calculate diffracted intensities. This is illustrated here using CaTiO 3 films deposited on SrTiO 3 substrates as a case study. By taking into account twinning in the structural analysis, it is shown that the structure of the CaTiO 3 films can be refined with an accuracy comparable to that obtained by dynamical refinement from non-twinned data. The introduction of the possibility to handle twin data sets is undoubtedly a valuable add-on and, notably, paves the way for a successful use of precession electron diffraction tomography for accurate structural analyses of thin films. 1. Introduction The development in materials science towards ‘nanomaterials’ challenges the diffraction techniques on their ability to char- acterize the crystal structure of an object with a small diffracting volume. The epitaxial thin films of functional oxides deposited on monocrystalline substrates are an illus- tration of this. Perovskite-derived materials are among the most widely studied and arouse constant interest because of their broad spectrum of properties such as ferroelectricity (Guilloux-Viry et al. , 2013; Thomasson et al., 2013; Trassin, Cherifi et al., 2015; Trassin, Luca et al., 2015), magnetism (Bai et al., 2005; Be ´a et al., 2007; Bhattacharya & May, 2014), incipient ferroelectricity (Haeni et al., 2004) and insulator– metal transitions (Ahn et al., 2004). In these materials, small structural changes induced by the application of strains can cause changes in the observed properties (Rondinelli et al., 2012). Often referred to as ‘strain engineering’ (Schlom et al., 2014), substrate-imposed epitaxial stress is one way to modify the ground state of epitaxial thin films in order to obtain new or heightened properties (Baek et al., 2011; Fan et al., 2014; Huang & Chen, 2014; Biegalski et al., 2015a,b). In most cases, the structure of the related bulk compound is well known and ISSN 1600-5767 # 2019 International Union of Crystallography