7 3 I H I G H L I G H T S 2 0 2 2
investigation is required to clearly identify the mechanisms governing the strain behaviour in MAPbI3 layers.
The texture in MAPbI3 layers was then investigated by means of synchrotron FFDXM a powerful technique that enables the direct mapping of crystalline lattices at beamline ID01, with the aim to understand the nature of a peculiar double lattice orientation that is sometimes observed in MAPbI3(Cl) instead of the more common unique orientation. The FFDXM results show that these textures are the signature of the presence of ferroelastic twins in the perovskite layer. Indeed, as shown in Figure 61, strong spatial and tilt correlations were evidenced between the distributions of the two [hh0] and [00l] types of grains, demonstrating the presence of twin domains in the grains, which form as the layer experiences a cubic-tetragonal phase transition at around 57°C. Their ferroelastic nature was demonstrated by the observation of the flipping of [00l]-domains into [hh0]-ones under the
PRINCIPAL PUBLICATION AND AUTHORS
Microstructure of Methylammonium Lead iodide Perovskite Thin Films: A Comprehensive Study of the Strain and Texture, A.A. Medjahed (a), T. Zhou (b), J.C. Alvarez Quiceno (c,d), P. Dally (e,f), P. Pochet (d), T.U. Schülli (g), D. Djurado (a), P. Reiss (a), S. Pouget (d), Adv. Energy Mater. 2103627 (2022); https:/doi.org/10.1002/aenm.202103627 (a) University Grenoble Alpes, CEA, CNRS, IRIG, SyMMES, Grenoble (France) (b) Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois (USA) (c) Instituto de Fisica, Universidad de Antioquia UdeA, Medellin (Colombia) (d) University Grenoble Alpes, CEA, IRIG, MEM, Grenoble (France) (e) Institut Lavoisier de Versailles (ILV), Univ. Versailles Saint-Quentin-en-Yvelines, Univ. Paris-Saclay, CNRS, Versailles (France) (f) IPVF, F-91120, Palaiseau (France) (g) ESRF
 D. Liu et al., Nat. Mater. 20, 1337-1346 (2021).  P. Zhao et al., Mater. Today Energy 17, 100481 (2020).
In-situ full-field diffraction X-ray microscopy reveals new insights into ferroelastic domain dynamics in perovskites
Ferroelastic domains in perovskites have been probed using different techniques, but investigating their local dynamics over crystal phase transitions is challenging since it requires a combination of good space and time resolution with high strain sensitivity. Full-field diffraction X-ray microscopy was used to reveal new properties of perovskite nanostructures.
Metal halide perovskites (MHPs) have shown impressive results in solar cells, light-emitting devices and scintillator applications, but questions regarding their complex structure are still open . The MHP CsPbBr3 forms ferroelastic domains upon crystal phase transitions 
or external stress , but probing the dynamics of such domains is challenging since standard techniques like transmission electron microscopy (TEM) require severe sample preparation and struggles to image large areas. Nanofocused scanning X-ray diffraction (nano-XRD) can combine high strain sensitivity and space resolution but presents poor time resolution. Full-field diffraction X-ray microscopy (FFDXM), recently available at beamline ID01, offers the possibility of in-situ and operando measurements of large crystals with good space and time resolution (Figure 62a).
In this work, FFDXM was used to image the domain evolution of a single crystal CsPbBr3 nanoplatelet at 150 nm and 6 s resolution. In contrast with smooth variations on the overall domain shape and orientations along the crystal from room temperature up to 70°C (Figure 62b), local sudden domain changes could be
synchrotron beam, most likely due to the stress induced by the ion migration experienced by the material when exposed to the beam.
The stability of the different domain orientations was then investigated by density functional theory (DFT) calculations. PbI2-terminated surfaces were found to favour the [hh0] orientations while, for MAI-terminated ones, both [hh0] and [00l] domains were equally stabilised. This shows that the chemical nature of the terminations can drive the orientation of the crystallites, illustrating the determining role of the chemical environment at the film- substrate interface.
Taken together, these different results constitute an important advance in understanding the microstructure of hybrid halide perovskite layers, paving the way to methods of engineering them to enhance their optoelectronic properties, device performance and stability.