We report transient highly strained structural states in individual palladium (Pd) nanocrystals, electronically heated using an optical laser, which precede their uniform thermal expansion. Using an X-ray free-electron laser probe, the evolution of individual 111 Bragg peaks is measured as a function of delay time at various laser fluences. Above a laser fluence threshold at a sufficient pump-probe delay, the Bragg peak splits into multiple peaks, indicating heterogeneous strain, before returning to a single peak, corresponding to even heat distribution throughout the lattice expanded crystal. Our findings are supported by a lattice displacement and strain model of a single nanocrystal at different delay times, which agrees with the experimental data. Our observations have implications for understanding femtosecond laser interactions with metals and the potential photo-catalytic performance of Pd.

Macromolecular crowding plays a crucial role in modulating protein dynamics in cellular and in vitro environments. Polymeric crowders such as dextran and Ficoll are known to induce entropic forces, including depletion interactions, that promote structural organization, yet their nanoscale consequences for protein dynamics remain poorly understood. Here, we employ megahertz X-ray photon correlation spectroscopy (MHz-XPCS) at the European X-ray Free Electron Laser (XFEL) to probe the dynamics of the protein ferritin in solutions containing sucrose, dextran, and Ficoll. We find pronounced changes in collective protein dynamics in polymeric crowders, revealing depletion-driven short-range attractions that, combined with long-range repulsions, give rise to intermediate-range organization. These mesoscale correlations undergo collective relaxation on microsecond to millisecond timescales, as directly resolved by XPCS through the decay of ferritin density fluctuations. The magnitude of this depends sensitively on crowder molecular weight and type. Normalizing the crowder concentration by c* reveals scaling behavior of ferritin self-diffusion with a crossover near 2c*, marking a transition from depletion-enhanced mobility to viscosity-dominated slowing. Our results demonstrate that bulk properties alone are insufficient to describe protein dynamics in crowded solutions, highlighting the need to include polymer-specific interactions and depletion theory in models of crowded environments.

We present a split-and-delay study on an aqueous salt solution. We employed SACLA split-and-delay optics to split each FEL pulse into pair of two pulses and delayed one of them. The sample was delivered via a liquid jet system which was illuminated by the double-pulses as a function of the delay time from 50 fs to 1 ps. The results indicate beam induced heating effects in the sample from 250 fs.

Monochromatization and diagnostics of SASE spectral properties over the entire hard X-ray range are important for many experiments utilizing transverse and longitudinal coherence of XFEL beam. For this purpose the Materials Imaging and Dynamics instrument at European XFEL is equipped with two cryo-cooled double-crystal monochromators using Si111 and Si220 optics. Optical parameters, design and performance of both monochromators are presented.

Dark field x-ray microscopy (DXFM) can visualize microstructural distortions in bulk crystals. Using the femtosecond x-ray pulses generated by x-ray free-electron lasers (XFELs), DFXM can achieve sub-μm spatial resolution and <100 fs time resolution simultaneously. In this paper, we demonstrate ultrafast DFXM measurements at the European XFEL to visualize an optically driven longitudinal strain wave propagating through a diamond single crystal. We also present two DFXM scanning modalities that are new to the XFEL sources: spatial 3D and 2D axial-strain scans with sub-μm spatial resolution. With this progress in XFEL-based DFXM, we discuss new opportunities to study multi-timescale spatiotemporal dynamics of microstructures.

Mössbauer spectroscopy is widely used to study structure and dynamics of matter with remarkably high energy resolution, provided by the narrow nuclear resonance line widths. However, the narrow width implies low count rates, such that experiments commonly average over extended measurement times or many x-ray pulses (“shots”). This averaging impedes the study of non-equilibrium phenomena. It has been suggested that X-ray free-electron lasers (XFELs) could enable Mössbauer single-shot measurements without averaging, and a proof-of-principle demonstration has been reported. However, so far, only a tiny fraction of all shots resulted in signal-photon numbers which are sufficiently high for a single-shot analysis. Here, we demonstrate coherent nuclear-forward-scattering of self-seeded XFEL radiation, with up to 900 signal-photons per shot. We develop a sorting approach which allows us to include all data on a single-shot level, independent of the signal content of the individual shots. It utilizes the presence of different dynamics classes, i.e. different nuclear evolutions after each excitation. Each shot is assigned to one of the classes, which can then be analyzed separately. Our approach determines the classes from the data without requiring theory modeling nor prior knowledge on the dynamics, making it also applicable to unknown phenomena. We envision that our approach opens up new grounds for Mössbauer science, enabling the study of out-of-equilibrium transient dynamics of the nuclei or their environment.

The Multiple Detector Stage is an ancillary detector setup for the Materials Imaging and Dynamics instrument at the European X-Ray Free-Electron Laser Facility. It is developed to improve the current capabilities concerning X-ray detection and make entirely new experiments possible. A unique feature of the MID instrument is the large flexibility in positioning of the AGIPD detector relative to the sample. This enables a large variety of instrument configurations ranging from small-angle to wide-angle X-ray scattering setups. A recurrent request from the users, which is currently not enabled, is the option of simultaneously recording both wide- and the small angle scattering by using two area detectors. The aim of developing MDS is to provide this missing capability at MID so that SAXS and WAXS experiments can be performed in parallel. The MDS will not be installed permanently at the instrument but only on request to provide as much flexibility as possible. In this article, the background and status of the MDS project is described in detail.

An experimental setup for liquid jet applications was implemented at the Materials Imaging and Dynamics Instrument (MID) of the European XFEL. This setup is operated in the multi-purpose experimental chamber of MID at pressures down to 10 − 5 mbar. The motion of the nozzle was realized in three dimensions for alignment purposes and the investigation of liquids at different states of supercooling. For this purpose, an active nozzle temperature control is implemented and a catcher system has been installed to capture the liquid after passing the interaction region to maintain good vacuum conditions. The setup is compatible with different nozzle designs. For spill protection, additional housing with secondary pumping around the nozzle, liquid jet and catcher was implemented. A magnifying camera system for aligning the jet and shadow imaging is available. The setup has already been commissioned and operated with different jets and detector configurations.

Material processing with femtosecond lasers has attracted enormous attention because of its potential for technology and industrial applications. In parallel, time-resolved x-ray diffraction has been successfully used to study ultrafast structural distortion dynamics in semiconductor thin films or surface layers. However, real-world processing applications mostly are concerned with bulk materials, which prevents the use of x-ray surface based techniques. For processing applications, a fast and depth-sensitive probe is needed. To address this, we present a novel technique based on ultrafast x-ray dynamical diffraction (UDD) capable of imaging transient strain distributions inside bulk crystals upon laser excitation. This pump-probe technique provides a complete picture of thetemporal evolution of ultrafast distorted lattice depth profiles. We demonstrate the potential of UDD by studying a thin Si single crystal upon single pulse femtosecond optical excitation. Our study reveals that below the melting threshold strong lattice distortions not only longitudinal, but also transversal to the propagation of the strain wave appear on picosecond time scales along the single crystal. The observation of this transversal deformation after laser excitation contradicts previous work that were not able to observed it, what could be related to the high sensitivity of dynamical diffraction with respect to the lattice distortions. The speed of propagation of this ultrafast transversal strain deformation is observed to be slower to the longitudinal sound speed for Si as described in the bibliography.

The availability of intense X-ray beams at megahertz repetition rate allows for new scattering and imaging experiments using the unique pulse train structure of the European X-ray Free-Electron Laser (EuXFEL). However, the resulting heat load can pose challenges for X-ray optics and potentially limit the efficiency. In this context, X-ray optics made of diamond emerge as a promising solution better suited for the extreme beam conditions at EuXFEL. In this article, we demonstrate a simple speckle analysis to determine the size of the focused beam in a caustic scan. The lenses are 1D planar diamond lenses and experiments were performed at the Materials Imaging and Dynamics (MID) station of EuXFEL. We find a minimum beam size of 630 nm and compare the speckle method with the more conventional method of wire scanning.

Understanding protein motion within the cell is crucial for predicting reaction rates and macromolecular transport in the cytoplasm. A key question is how crowded environments affect protein dynamics through hydrodynamic and direct interactions at molecular length scales. Using megahertz X-ray Photon Correlation Spectroscopy (MHz-XPCS) at the European X-ray Free Electron Laser (EuXFEL), we investigate ferritin diffusion at microsecond time scales. Our results reveal anomalous diffusion, indicated by the non-exponential decay of the intensity autocorrelation function g 2 ( q , t ) at high concentrations. This behavior is consistent with the presence of cage-trapping between the short- and long-time protein diffusion regimes. Modeling with the δ γ -theory of hydrodynamically interacting colloidal spheres successfully reproduces the experimental data by including a scaling factor linked to the protein direct interactions. These findings offer insights into the complex molecular motion in crowded protein solutions, with potential applications for optimizing ferritin-based drug delivery, where protein diffusion is the rate-limiting step.

The phase-seeding method proposed by Carrozzini et al. [(2025), Acta Cryst. A81, 188-201] introduces a strategy for integrating artificial intelligence (AI) with established ab initio phasing techniques. Rather than presenting an AI-based phasing solution itself, the authors demonstrate how traditional crystallographic methods can be significantly enhanced if provided with a small subset of approximate phase values - a `phase seed' - that could, in principle, be generated by a machine learning model. By discretizing phase values into a few angular bins, the method transforms the continuous phase problem into a classification task, thereby reducing the computational burden on AI training. This hybrid approach shows promise for improving structure solution, particularly for large and complex non-centrosymmetric crystals, and opens a pathway for future AI-assisted crystallographic workflows.

The process of merging bubbles or droplets with one another or with a continuous phase is known as coalescence.⁠1 It takes place from the microscale, e.g., in technological applications such as contrast-enhanced ultrasound in medicine, to the macroscale, e.g., in planet and star formation. Coalescence of water droplets is an important process in nature, e.g., in Earth’s troposphere for growth of raindrops⁠2. Usually coalescence is studied at ambient temperature, but even in clouds it takes place in the temperature range down to 235 K, where water is supercooled and metastable with respect to crystalline ice.⁠3 In space, transport of molecules between water interfaces takes place at even harsher conditions in the deeply supercooled or glassy state, down to 10 K. This is for example the case for grainy amorphous ice covering interstellar dust particles in molecular clouds⁠4. Yet, it is currently unknown whether droplet coalescence occurs in deeply supercooled water at all and if so at what time scale. Here we observe that micrometer-sized glassy water droplets coalesce between ~123 and ~140 K upon slow heating on the time scale of hours to minutes based on small-angle X-ray scattering as well as scanning electron microscopy experiments. Droplet interfaces start to vanish close to water’s first glass transition temperature, indicating that water molecules experience translational motion across the droplet interfaces even under cryo-conditions. This means that glassy low-density water turns into a viscous liquid at its glass transition temperature, ruling out the possibility of an orientational glass transition⁠5 or point defect dynamics⁠6. The latter cases would not lead to a supercooled liquid but to a solid with rotational disorder. This finding helps to resolve the debate of whether amorphous ice is thermodynamically continuously connected to deeply supercooled liquid water in terms of a glass transition⁠7–12 and provides us with an ultraslow-motion observation of the processes of coalescence at high viscosities. The idea in coalescence theory13 that the viscous force arrests the droplet interfaces does not hold at 125 K, where water is of ultrahigh viscosity, but coalescence takes place.

Phase transitions are governed by both intrinsic and extrinsic heterogeneities, yet capturing their spatio-temporal dynamics remains a challenge. While ultrafast techniques track phase changes on femtosecond timescales, the spatial complexity and stochastic nature of the processes often remain hidden. Here, we present an experimental approach that combines well-established ultrafast hard-X-ray diffraction with a propagating strain pulse as a universal and non-invasive probe. This ultrafast X-ray sonography can capture the spatio-temporal phase heterogeneity in great detail by resolving the phase-specific strain response. We apply this approach to the antiferromagnetic-to-ferromagnetic magneto-structural phase transition in FeRh and identify the ferromagnetic phase to nucleate at the surface as narrow columnar domains of approximately 30 nm diameter. Besides reconciling the diverse experimental results in the literature on FeRh, X-ray sonography offers a versatile platform for investigating a wide range of phase transitions accompanied by structural changes.

The development of organic metal salt hydrates as supramolecular crystalline materials requires a fundamental understanding of how molecular assembly, involving diverse types of lattice-bound water, can influence both microstructure and macroscopic properties. Herein, we demonstrate electron diffraction as a powerful tool for capturing the structure of an elusive nanosized organic sodium salt hydrate with variable water content, using the drug compound risedronate sodium (RS) as a case study. Within the complex structure of the 2.5-hydrate featuring ion-coordinated, channel, and isolated-site water molecules, we found that dehydration unexpectedly occurs at ion-associated water molecules rather than at hydrogen-bonded ones. Combining electron diffraction, synchrotron powder X-ray diffraction, and noncovalent interaction analysis based on density-functional theory, we elucidate the underlying transition mechanism resulting from motions of lattice water and conclude principles of water site removal. Our findings reveal that the fast interconversion between 2.5-hydrate and a nonstoichiometric hydrate arises from their dynamic, reversible noncovalent interactions related to water molecules and a flexible sodium ion-coordination network. This study provides an example of applying electron diffraction to elucidate the structure of short-lived, nonstoichiometric organic metal salt hydrates and sheds light on how the hydrate structures with different water-ion coordination impact the supramolecular organization and dehydration pathways.

The Debye-Waller factor, introduced a century ago, remains a fundamental component in the refinement of crystal structures against X-ray, neutron and electron diffraction data. This review marks its centenary by exploring its applications in small-molecule crystallography. We provide a historical overview of the development of the Debye-Waller factor and its foundations in lattice dynamics. The review discusses the practical use of anisotropic displacement parameters and their role in accurate structure determination. We also address the challenges and advancements in modelling thermal motion and disorder, the role of multi-temperature measurements and modern computational approaches.

Understanding and ultimately controlling the transformations and properties of nanoscale systems, from proteins to synthetic nanomaterial assemblies, is limited by the inability to uncover their dynamics on their characteristic length and time scales. Here, we nevertheless demonstrate this ability using MHz X-ray photon correlation spectroscopy (XPCS) – directly elucidating the characteristic microsecond-dynamics of density fluctuations of semiconductor nanocrystals (NCs), not only in a colloidal dispersion but also in a liquid phase consisting of densely packed, yet mobile, NCs with no long-range order. We find the wavevector-dependent fluctuation rates in the liquid phase are suppressed relative to those in the colloidal phase and relative to observations of densely packed repulsive particles. We show that the suppressed rates are due to a substantial decrease in the self-diffusion of NCs, which we attribute to explicit attractive interactions. Using coarse-grained simulations, we find that the extracted shape and strength of the interparticle potential explains the stability of the liquid phase, in contrast to the gelation observed via XPCS in many other charged colloidal systems. This work opens the door to elucidating fast, condensed phase dynamics in complex fluids and other nanoscale soft matter, such as densely packed proteins and non-equilibrium self-assembly processes, in addition to designing microscopic strategies to avert gelation.

With the continued relevance of drug hydrates in pharmaceutical sciences, a comprehensive understanding of hydrate and anhydrate forms is essential, not only through individual case studies but also from a broader, systematic perspective. The Cambridge Structural Database (CSD) is a well-established database for crystal structures of organic molecules and here, the structural features of pharmaceutically relevant compounds forming hydrates were explored. Drug anhydrate and hydrate subsets were generated and further classified into separate anhydrate and hydrate sets for free drug, cocrystal/solvate, salt, multicomponent cocrystal/solvate, and salt cocrystal/solvate systems. A thorough understanding of these sets was documented at molecular and structural levels. The CSD drug subset contains 24% of entries as hydrates and 76% as anhydrates. Only 6% of anhydrates have corresponding hydrate forms in the CSD drug subset. The formation of hydrates seems to be still less documented in multicomponent drug hydrates, as well as polymorphism of hydrates is less explored for these increasingly complicated systems with a high number of components. The presence of water molecules or additional components does not necessarily lead to a higher degree of crystal packing. Water is involved in 44% of hydrogen bonds (H-bond) in drug hydrate set, where water prefers to act as H-bond donor. H-bonds formed only by water show a relatively high bond strength. This work demonstrates the potential of data science in analyzing pharmaceutically relevant databases to uncover hidden patterns, and more specifically utilizing the CSD for understanding structural aspects and the role of water in H-bond patterns in drug hydrates.