Defects and their interactions in crystalline solids often underpin material properties and functionality 1 as they are decisive for stability 1-5 , result in enhanced diffusion 6 , and act as a reservoir of vacancies 7 . Recently, lithium-rich layered oxides have emerged among the leading candidates for the next-generation energy storage cathode material, delivering 50 % excess capacity over commercially used compounds. Oxygen-redox reactions are believed to be responsible for the excess capacity 8 , however, voltage fading has prevented commercialization of these new materials. Despite extensive research the understanding of the mechanisms underpinning oxygen-redox reactions and voltage fade remain incomplete. Here, using operando three-dimensional Bragg coherent diffractive imaging 2,9 , we directly observe nucleation of a mobile dislocation network in nanoparticles of lithium-rich layered oxide material. Surprisingly, we find that dislocations form more readily in the lithium-rich layered oxide material as compared with a conventional layered oxide material, suggesting a link between the defects and the

High catalytic efficiency in metal nanocatalysts is attributed to large surface area to volume ratios and an abundance of under-coordinated atoms that can decrease kinetic barriers. Although overall shape or size changes of nanocatalysts have been observed as a result of catalytic processes, structural changes at low-coordination sites such as edges, remain poorly understood. Here, we report high-lattice distortion at edges of Pt nanocrystals during heterogeneous catalytic methane oxidation based on in situ 3D Bragg coherent X-ray diffraction imaging. We directly observe contraction at edges owing to adsorption of oxygen. This strain increases during methane oxidation and it returns to the original state after completing the reaction process. The results are in good agreement with finite element models that incorporate forces, as determined by reactive molecular dynamics simulations. Reaction mechanisms obtained from in situ strain imaging thus provide important insights for improving catalysts and designing future nanostructured catalytic materials.

We show that the combination of X-ray scattering with a nanofocused beam and X-ray cross correlation analysis is an efficient means for the full structural characterization of mesocrystalline nanoparticle assemblies with a single experiment. We analyze several hundred diffraction patterns of individual sample locations, i.e. individual grains, to obtain a meaningful statistical distribution of the superlattice and atomic lattice ordering. Simultaneous small-and wide-angle X-ray scattering of the same sample location allows us to determine the structure and orientation of the superlattice as well as the angular correlation of the first two Bragg peaks of the atomic lattices, their orientation with respect to the superlattice, and the average orientational misfit due to local structural disorder. This experiment is particularly advantageous for synthetic mesocrystals made by the simultaneous self-assembly of colloidal nanocrystals and surfacefunctionalization with conductive ligands. While the structural characterization of such materials has been challenging so far, the present method now allows correlating mesocrystalline structure with optoelectronic properties. Mesocrystals (MC) are three-dimensional arrays of iso-oriented single-crystalline particles with an individual size between 1 -1000 nm. [1][2][3][4][5] Their physical properties are largely determined by structural coherence, for which the angular correlation between their individual atomic lattices and the underlying superlattice of nanocrystals (NC) is a key ingredient. 1,2 Colloidal NCs stabilized by organic surfactants have been shown to pose excellent building blocks for the design of synthetic MCs with tailored structural properties which are conveniently obtained by self-assembly of NCs from solution on a solid or liquid substrate by exploiting ligand-ligand 3 interactions. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] Typically, the utilized ligands consist of wide-gap, bulky hydrocarbons which render the MCs insulating. [26][27][28][29][30][31][32][33] MCs obtained in this way exhibit average grain sizes of ~150 µm 2 , which enables a detailed characterization by electron and/or X-ray microscopy. 34 Since the optoelectronic properties of PbS NC ensembles bear many opportunities for applications in solar cells or photodetectors, a number of ligand exchange procedures with small organic or inorganic molecules as well as single atom passivation strategies have been developed, all of which greatly increase the carrier mobilities within the SL of NCs. 28,33,[35][36][37][38][39][40][41][42][43][44] Due to the short interparticle spacing imposed by these ligands, structural coherence is mostly lost in such superlattices, but in rare cases it has been demonstrated that significant long-range order and even mesocrystallinity can be preserved. 25,35,45 However, a persisting problem of these protocols is that they are prone to introduce defects in the superlattice structure with some degree of granularity and signifi...

We have used scanning micro x-ray diffraction to characterize different phases in superconducting KxFe2−ySe2 as a function of temperature, unveiling the thermal evolution across the superconducting transition temperature (Tc ∼32 K), phase separation temperature (Tps ∼520 K) and iron-vacancy order temperature (Tvo ∼580 K). In addition to the iron-vacancy ordered tetragonal magnetic phase and orthorhombic metallic minority filamentary phase, we have found a clear evidence of the interface phase with tetragonal symmetry. The metallic phase is surrounded by this interface phase below ∼300 K, and is embedded in the insulating texture. The spatial distribution of coexisting phases as a function of temperature provides a clear evidence of the formation of protected metallic percolative paths in the majority texture with large magnetic moment, required for the electronic coherence for the superconductivity. Furthermore, a clear reorganization of iron-vacancy order around the Tps and Tc is found with the interface phase being mostly associated with a different iron-vacancy configuration, that may be important for protecting the percolative superconductivity in KxFe2−ySe2.

We present an x-ray study of freely suspended hexatic films of the liquid crystal 3(10)OBC.Our results reveal spatial inhomogeneities of the bond-orientational (BO) order in the vicinity of the hexatic-smectic phase transition and the formation of large scale hexatic domains at lower temperatures. Deep in the hexatic phase up to 25 successive sixfold BO order parameters have been directly determined by means of angular x-ray cross-correlation analysis (XCCA). Such strongly developed hexatic order allowed us to determine higher order correction terms in the scaling relation predicted by the multicritical scaling theory over a full temperature range of the hexatic phase existence. 1The influence of angular correlations on structural and physical properties of complex fluids, colloidal suspensions and liquid crystals (LCs) remains one of fundamental and unresolved problems in modern condensed matter physics [1]. A prominent example of a system with angular correlations is the hexatic phase that combines the properties of both crystals and liquids [2]. The two-dimensional (2D) hexatic phase shows a sixfold quasi-long range bond-orientational (BO) order, while the positional order is short range [3]. The hexatic phase is a general phenomenon that was observed in a number of systems of various physical nature, such as 2D colloids [4][5][6], electrons at the surface of helium [7], 2D superconducting vortexes [8,9] and, particularly, in liquid crystals [10][11][12][13].The hexatic phase was predicted by Halperin and Nelson [14] as an intermediate state in 2D crystal melting. According to their theory the hexatic phase arises as a consequence of the broken translational symmetry of a 2D crystal induced by dissociation of dislocation pairs.This mechanism does not work in 3D crystals, however, the 3D hexatic phase was observed experimentally in LCs [10]. The multicritical scaling theory (MCST) developed by Aharony and coworkers [15] based on renormalization group approach to critical phenomena enabled quantitative characterization of the BO order in the hexatic phase and, particularly, allowed to study a crossover from 2D to 3D behavior [11,16]. In spite of the extensive experimental and theoretical work the origin of the hexatic phase in LCs and the features of the hexatic -smectic phase transition remain puzzling and controversial.The structure of hexatics is traditionally studied by means of x-ray or electron diffraction in a single-domain area of a hexatic film (see for reviews [17][18][19]). The quantitative characteristics of the BO order, the so-called BO order parameters [12], are typically determined by fitting the measured azimuthal intensity distribution by the Fourier cosine series. In contrast to this approach in the present work we performed spatially resolved x-ray diffraction studies of free standing LC films. Measured x-ray data were analyzed by means of direct Fourier transformation and by using angular x-ray cross-correlation analysis (XCCA) [20][21][22][23]. The latter method enabled a direct d...

The Materials Imaging and Dynamics (MID) instrument at the European X-ray Free-Electron Laser (EuXFEL) facility is described. EuXFEL is the first hard X-ray free-electron laser operating in the MHz repetition range which provides novel science opportunities. The aim of MID is to enable studies of nano-structured materials, liquids, and soft- and hard-condensed matter using the bright X-ray beams generated by EuXFEL. Particular emphasis is on studies of structure and dynamics in materials by coherent scattering and imaging using hard X-rays. Commission of MID started at the end of 2018 and first experiments were performed in 2019.

A setup is presented for automated high-throughput measurements of smallangle X-ray scattering (SAXS) from macromolecular solutions on the bendingmagnet beamline X33 of EMBL at the storage ring DORIS-III (DESY, Hamburg). A new multi-cell compartment allows for rapid switching between in-vacuum and in-air operation, for digital camera assisted control of cell filling and for colour sample illumination. The beamline is equipped with a Pilatus 1 M-W pixel detector for SAXS and a Pilatus 300 k-W for wide-angle scattering (WAXS), and results from the use of the Pilatus detectors for scattering studies are reported. The setup provides a broad resolution range from 100 to 0.36 nm without the necessity of changing the sample-to-detector distance. A new optimized robotic sample changer is installed, permitting rapid and reliable automated sample loading and cell cleaning with a required sample volume of 40 ml. All the devices are fully integrated into the beamline control software system, ensuring fully automated and user-friendly operation (attended, unattended and remote) with a throughput of up to 15 measurements per hour.

We present an x-ray study of liquid crystal membranes in the vicinity of the hexatic-smectic phase transition by means of angular x-ray cross-correlation analysis. By applying two-point angular-intensity cross-correlation functions to the measured series of diffraction patterns the parameters of bond-orientational (BO) order in hexatic phase were directly determined. The temperature dependence of the positional correlation lengths was analyzed as well. The obtained correlation lengths show larger values for the higher-order Fourier components of BO order. These findings indicate a strong coupling between BO and positional order. About 30 years ago it was realized that the phase transition between a two-dimensional (2D) crystal and the liquid phase can proceed through an intermediate hexatic phase [1]. The corresponding mechanism involves dissociation of dislocation pairs. The 2D hexatic phase is characterized by a sixfold quasilong-range bond-orientational (BO) order, while the positional order is short range and the shear modulus is zero. Phases with hexatic order have been found in several systems, such as electrons at the surface of helium [2], charged polymer colloids [3,4], and smectic liquid crystals (LCs) [5][6][7]. In the last case a hexatic phase was experimentally observed quite unexpectedly in three-dimensional (3D) stacked molecular systems [8]. In the 3D hexatic phase the positional order is short range, while the BO order persists over a long range. However, the mere existence of a hexatic phase does not imply a specific melting mechanism, and the origin of hexatic order and its relation to the defect-mediated melting transition is still controversial. Smectic liquid crystals are particularly suitable to investigate these problems, as they can be suspended over an opening in a solid frame. Such smectic membranes are substrate-free and have a controlled thickness ranging from two to more than thousands of layers [9].Smectic-A membranes can be described as stacks of liquid layers. The in-plane structure is liquidlike with positional correlations between the molecules decaying exponentially with a correlation length ξ 0 . While cooling a hexatic phase may occur, which shows a long-range BO order. This leads to a sixfold rotational symmetry and the BO correlations are characterized by the local ordering field ψ(r) ∝ exp[i6θ (r)], where θ (r) is the angle between the "bonds" and some reference axis. Upon decreasing the temperature, the width of the radial-intensity peak decreases simultaneously with a further development of the BO order. This indicates a coupling * Present address: The University of California, San Diego, La Jolla, CA 92093, USA.† Corresponding author: ivan.vartaniants@desy.de between the positional correlations and the BO order [10]. Additionally, at even lower temperatures a 3D crystalline phase appears with a hexagonal in-plane lattice or a rectangular lattice with a so-called herringbone order [6,7]. The common way to study the BO order is to perform x-ray or electron diffraction me...