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Fig. 63: a) Sketch of the three-dimensional domain structure, with arrows indicating the directions of the spontaneous polarisation within each domain. b) Sketch showing the alternating in-plane and out-of-plane flux- closure domains that lead to the distortions summarised in (c). d) Top: High-resolution scanning transmission electron microscopy image of the interfaces between PbTiO3 and SrRuO3 showing the bending of the lattice. Bottom: Local curvature in each atomic plane. Figure adapted from principal publication with permission from Springer Nature.

Importantly, the coupling between the different ferroelectric layers results in periodic bending of the SrRuO3 spacer layers in order to accommodate the deformation of the PbTiO3 layers. This imposes very large curvatures (of the order of 107 m-1) that lead to large strain gradients in the metallic layers (Figures 63c and 63d). Strain gradients naturally break inversion symmetry in any material and can lead to a range of novel properties, including large

flexoelectric response in semiconductors and interesting transport properties in metallic systems. Because the curvature of the spacer layers is induced by the ferroelectric domain structure, it can be modified using electric fields. Such supercrystals, combining ferroelectric materials with different spacer materials, provide a new way to manipulate the electronic structure of oxides and to explore a variety of strain gradient-induced phenomena.

PRINCIPAL PUBLICATION AND AUTHORS

Metal-ferroelectric supercrystals with periodically curved metallic layers, M. Hadjimichael (a,b), Y. Li (a), E. Zatterin (a,c), G.A. Chahine (c,d), M. Conroy (e), K. Moore (e), E.N. O Connell (e), P. Ondrejkovic (f), P. Marton (f), J. Hlinka (f), U. Bangert (e), S. Leake (c), P. Zubko (a), Nat. Mater. 20, 495-502 (2021); https:/doi.org/10.1038/s41563-020-00864-6 (a) London Centre for Nanotechnology and Department of Physics and Astronomy, University College London (UK) (b) Department of Quantum Matter Physics, University of Geneva (Switzerland) (c) ESRF (d) Université Grenoble Alpes, CNRS, Grenoble (France) (e) Department of Physics, Bernal Institute, University of Limerick (Ireland) (f) Institute of Physics of the Czech Academy of Sciences, Prague (Czech Republic)

REFERENCES

[1] G. Catalan et al., Rev. Mod. Phys. 84, 119-156 (2012). [2] J. Íñiguez et al., Nat. Rev. Mater. 4, 243-256 (2019). [3] M. Hadjimichael et al., Phys. Rev. Lett. 120, 037602 (2018). [4] P. Zubko et al., Phys. Rev. Lett. 104, 187601 (2010). [5] M. Hadjimichael et al., Phys. Rev. Mater. 4, 094415 (2020).

Depth-profiling the Li intercalation in graphite electrodes for Li-ion batteries: model and experiment

Understanding and accurately simulating the Li distribution in electrodes is key for developing Li- ion technology. Combining micro-focused operando XRD experiments and numerical simulations, Li concentration was matched, measured and predicted across the depth of graphite electrodes. Unexpected behaviour was evidenced and successfully modelled for the larger Li concentration.

Li-ion batteries (LiB) are ubiquitous; they power a wide range of devices and are expected to play a key role in the incoming energy transition. In a LiB, two opposite electrodes (anode and cathode) host the Li ions, which shuttle between them during charge or discharge. Several physico-chemical processes dictate the behaviour of the Li ions as they travel from one electrode to the other such as liquid-phase diffusion in the electrolyte, insertion reaction at the electrode/electrolyte interface, and solid-phase diffusion in the electrode material. These processes have to be correctly described to achieve accurate modelling of the behaviour of the LiB, paving the way to predictive