Strontium iron oxide is a candidate for many different energy applications, including solid oxide fuel cells, chemical looping, and thermochemical energy storage. Upon the redox reactions, SrFeO_x cycles between two end forms: an oxygen deficient form SrFeO_2.5 with a brownmillerite structure and an oxidized form SrFeO_3-δ with a perovskite structure. Two intermediate structures are reported from ex situ and in situ X-ray and neutron powder diffraction [1–3]. However, in real applications, submicron sized crystals are used and X-ray and neutron diffraction techniques are not able to access structural information on an individual submicron crystal. In situ 3D electron diffraction (3D ED) performed on a transmission electron microscope (TEM) is the only way to obtain single crystal data on all structural changes occurring during the actual redox reactions. Due to the single-tilt design of the environmental holders combined with the complexity of these structures, in-zone electron diffraction and high resolution imaging on random crystallites are unrealistic, but 3D ED does not require in zone orientation and could thus be successfully applied to gather structural data on the different phases.

We performed in situ oxidation of a brownmillerite SrFeO_2.5 crushed single crystal upon heating in an oxygen atmosphere in TEM using the sealed commercial holder. By acquiring 3D ED data at different steps of the reaction, we confirmed the formation of the perovskite SrFeO_3-δ structure, which we were able to reduce back to brownmillerite in a hydrogen atmosphere. The obtained data allowed us to derive the structures formed at different reaction steps, including the intermediate phases, resulting in new information about their crystal structures and microstructures. In my talk, I will compare the results of in situ 3D ED with the published data from X-ray and neutron diffraction, discuss the limitations of the method, and the next steps in improving in situ 3D ED in gas environments.

[1] A. Maity, R. Dutta, B. Penkala, M. Ceretti, A. Letrouit-Lebranchu, D. Chernyshov, A. Perichon, A. Piovano, A. Bossak, M. Meven, W. Paulus (2015). J. Phys. D. Appl. Phys. 48, 504004. [2] D.D. Taylor, N.J. Schreiber, B.D. Levitas, W. Xu, P.S. Whitfield, E.E. Rodriguez (2016). Chem. Mater. 28, 3951–3960. [3] J.P.P. Hodges, S. Short, J.D.D. Jorgensen, X. Xiong, B. Dabrowski, S.M.M. Mini, C.W.W. Kimball (2000). J. Solid State Chem. 151, 190-209.

Keywords: brownmillerite; TEM; 3D ED; in situ

This work was supported by BOF 38689 - New method to acquire in situ information on crystal structures changed by chemical reactions

Timescales of dynamic processes in extended solids span many orders of magnitude owing to the large number of degrees of freedom. Additionally, scaling laws dictate that discrete temporal domains comprising the entire continuum consist of associated spatial domains within which specific dynamics are dominant. Ideally, one would be able to probe the entire spatiotemporal range on a single specimen spot with a single instrument. Modern TEMs are exceptionally versatile in this regard, providing access to spatial and energy ranges that span sub-Å to micrometres and sub-10 meV to 1,000s of eV, respectively. However, timescales of the associated physical phenomena span 100s of attoseconds (10^-18 s) to minutes and longer, a range that cannot be fully covered by even the fastest direct detectors. Indeed, dynamics faster than ~0.1 ms are largely inaccessible to detector-based TEM approaches.

Here, an overview will be provided of ongoing efforts aimed at pushing TEM temporal resolutions well beyond the limits imposed by detectors and by peak dose rates. Emphasis will be placed on laser-driven nanosecond single-shot and fs stroboscopic modalities, currently the two most widely used approaches (Figure 1) [1-3]. Common hardware configurations based on modified commercial TEM platforms will be described, and current state-of-the-art performance specifications will be discussed. This will be followed by a brief survey of discoveries and advances that have been made with imaging, diffraction, and spectroscopy. Particular focus will be placed on experiments that have led to deeper understanding of materials and to new physics [4]. The talk will conclude with a brief look toward new emerging approaches and expanded applications, such as pulsed-beam damage mitigation [5, 6].

[1] Park, S. T., Flannigan, D. J. & Zewail, A. H. (2011). J. Am. Chem. Soc. 133, 1730.

[2] Cremons, D. R., Plemmons, D. A. & Flannigan, D. J. (2016). Nat. Commun. 7, 11230.

[3] Plemmons, D. A., Suri, P. K. & Flannigan, D. J. (2015). Chem. Mater. 27, 3178.

[4] Barwick, B., Flannigan, D. J. & Zewail, A. H. (2009). Nature 462, 902.

[5] VandenBussche, E. J. & Flannigan, D. J. (2019). Nano Lett. 19, 6687.

[6] VandenBussche, E. J., Clark, C. P., Holmes, R. J. & Flannigan, D. J. (2020). ACS Omega 5, 31867.

At present report time resolved methods and measurements of rocking curves (RC) and reciprocal space maps (RSM) under external dynamic ultrasonic loads are described. These measurements were made by using adaptive X-Ray optic (ABXO) elements.

Conducting experiments with time resolution using x-ray and synchrotron radiation is one of the advanced modern scientific problems. Today, there are three main directions in the development of such experiments - the creation of new sources (synchrotrons and XFELs), the development of detecting equipment, and the rapid tuning of experimental parameters. The first two directions have gained significant development in recent years, and the last direction rests on the impossibility of quickly adjusting experimental parameters using existing goniometric systems. As a result, the existing hardware and methodological base practically does not cover the range of time resolutions from seconds to microseconds, in which many interesting physical processes occur.

One of possibilities to overcome these limitations of traditional approach is using of non-mechanical adaptive X-ray optic elements, such as X-ray acoustic resonators of longitudinal oscillations or bimorph piezo-actuators [1]. It allows fast and precise variation of X-ray diffraction parameters, varying the angular position of the X-ray beam and controlling its wavelength. An important feature of the method is the possibility of conducting experiments not only in laboratory conditions, but also at synchrotron stations.

The method has been successfully applied to the study of processes occurring in crystals under dynamic ultrasonic loads. Using this method, studies of a silicon crystal subjected to quasistatic mechanical load were carried out [2]. The studies of the evolution of the defective structure of lithium fluoride (LiF) and TeO₂ single crystals under the conditions of dynamic ultrasonic loading in a wide range of amplitudes have also been studied [3]. It is shown that the diffraction pattern (shape and FWHM of rocking curves) under the action of ultrasound can differ significantly from the original, and the proposed method allows monitoring its changes with a temporal resolution of up to 10 μs, inaccessible when using mechanical goniometric systems. Studies of the evolution of the defective structure using the new method showed its significant (at least 3 orders of magnitude on a laboratory source) superiority in speed over existing methods.

The technique was also successfully applied for conducting experiments in a three-crystal X-ray diffraction scheme. It was shown that with its help it is possible to carry out fast (several minutes even with laboratory X-ray source) measurements of the reciprocal space maps from the studied samples under dynamic ultrasonic loads, as well as studying the distribution of ultrasonic vibrations in resonator crystals.

The reported study was funded by RFBR and DFG 19-52-12029 and by RFBR according to the research project №18-32-20108.

1. Blagov A.E., Bikov A.S and etc // IET. 2016. № 5. С. 109
2. Eliovich I.A., Akkuratov V.I. and etc. // Crystallography reports, 2018, Vol. 63, № 5, p. 708
3. Blagov A.E., Pisarevskii Yu.V. and etc. // PSS. 2017. Vol. 59. № 5. p. 947.

Many phase transformations associated with solid-state precipitation look structurally simple, yet take place with great difficulty. Classic cases of surprisingly difficult phase transformations can be found in alloy systems forming the basis for a broad range of high-strength lightweight aluminium alloys. In these systems, the difficult nucleation of strengthening phases, which are usually semi-coherent, is often preceded by the easy nucleation of another phase with strong structural similarities, typically a coherent precipitate. It is therefore of interest to investigate the reasons behind the difficult transformation from coherent to semi-coherent precipitate phases.

Using scanning / transmission electron microscopy (S/TEM) techniques both ex situ and in situ, combined with atomic scale simulations (density functional theory and semi-empirical potentials) we examined phase transformations in several alloy systems, including the textbook Al-Cu and Al-Ag systems. We show that certain microalloying additions, or different processing conditions applied to samples in bulk or nanoscale form, result in previously unreported precipitate phases [1-2] – see Figs. 1-2, or promote the nucleation of existing phases [3-4]. The nucleation mechanisms of these phases involve structural templates provided by coherent precipitates [1-3] and depend critically on the availability of vacancies [1-2,4]. Based on our observations atomic-scale mechanisms are proposed for phase transformation pathways. We also characterised the surface structure and growth mechanisms of voids, uncovering a crystallographic relationship necessary for the growth of high-aspect ratio voids [5]. These findings suggest several approaches to not only stimulate known precipitate transformations, but also discover new phases and transformation pathways.

The sensitiveness to the electron beam of nanocrystalline metal-organic frameworks (MOFs) has always posed an objective criticality for the accurate determination of their structure by single crystal electron diffraction. The reversible atomic displacement caused by the high flexibility of the organic linkers further complicates the characterization of the framework and the understanding of their complex functional properties at the atomic level ^[1]. Although standard diffraction experiments can elucidate dynamic phenomena ^[2,3], an analysis of the anisotropic displacement parameters (ADPs) obtained after refining MOFs against electron diffraction data has never been performed. In this study, we solved and refined the structures of UiO-67 /MIL-140C, coexisting in mixture, by using continuous rotation electron diffraction (cRED). For both structures, restricted small-angle librations of the linker were revealed by analysing the ADPs at room temperature and in cryogenic conditions (98 K). Our work shows that continuous rotation electron diffraction (cRED) not only provides reliable and accurate crystallographic models as that obtained by single crystal X- ray diffraction (SCXRD), but it represents a powerful tool to investigate the dynamic in the molecular fragments of the framework.

[1] Bennett, T.; Cheetham, A.; Fuchs, A.; Coudert, F.-X. (2017) Nature Chem., 9, 11–16

[2] Smeets, S.; Parois, P.; Burgi, H- B; Lutz, M. (2011) Acta Cryst, B67, 53-62

[3] Lock, N.; Wu, Y.; Christensen, M.; Cameron, L. J.; Peterson, V. K.; Bridgeman, A. J.; Kepert, C. J.; Iversen, B. B. (2010) J. Phys. Chem. C, 114, 16181− 16186