Comprehensive Application of XFEL Microcrystallography for Challenging Targets in Various Organic Compounds.

There is a growing demand for structure determination from small crystals, and the three-dimensional electron diffraction (3D ED) technique can be employed for this purpose. However, 3D ED has certain limitations related to the crystal thickness and data quality. We here present the application of serial X-ray crystallography (SX) with X-ray free electron lasers (XFELs) to small (a few µm or less) and thin (a few hundred nm or less) crystals of novel compounds dispersed on a substrate. For XFEL exposures, two-dimensional (2D) scanning of the substrate coupled with rotation enables highly efficient data collection. The recorded patterns can be successfully indexed using lattice parameters obtained through 3D ED. This approach is especially effective for challenging targets, including pharmaceuticals and organic materials that form preferentially oriented flat crystals in low-symmetry space groups. Some of these crystals have been difficult to solve or have yielded incomplete solutions using 3D ED. Our extensive analyses confirmed the superior quality of the SX data regardless of crystal orientations. Additionally, 2D scanning with XFEL pulses gives an overall distribution of the samples on the substrate, which can be useful for evaluating the properties of crystal grains and the quality of layered crystals. Therefore, this study demonstrates that XFEL crystallography has become a powerful tool for conducting structure studies of small crystals of organic compounds.

In situ high-pressure pair distribution function measurement of liquid and glass by using 100 keV pink beam.

Understanding the pressure-induced structural changes in liquids and amorphous materials is fundamental in a wide range of scientific fields. However, experimental investigation of the structure of liquid and amorphous material under in situ high-pressure conditions is still limited due to the experimental difficulties. In particular, the range of the momentum transfer (Q) in the structure factor [S(Q)] measurement under high-pressure conditions has been limited at relatively low Q, which makes it difficult to conduct detailed structural analysis of liquid and amorphous material. Here, we show the in situ high-pressure pair distribution function measurement of liquid and glass by using the 100 keV pink beam. Structures of liquids and glasses are measured under in situ high-pressure conditions in the Paris-Edinburgh press by high-energy x-ray diffraction measurement using a double-slit collimation setup with a point detector. The experiment enables us to measure S(Q) of GeO2 and SiO2 glasses and liquid Ge at a wide range of Q up to 20-29 Å-1 under in situ high-pressure and high-temperature conditions, which is almost two times larger than that of the conventional high-pressure angle-dispersive x-ray diffraction measurement. The high-pressure experimental S(Q) precisely determined at a wide range of Q opens the way to investigate detailed structural features of liquids and amorphous materials under in situ high-pressure and high-temperature conditions, as well as ambient pressure study.

A versatile pressure-cell design for studying ultrafast molecular-dynamics in supercritical fluids using coherent multi-pulse x-ray scattering.

Supercritical fluids (SCFs) can be found in a variety of environmental and industrial processes. They exhibit an anomalous thermodynamic behavior, which originates from their fluctuating heterogeneous micro-structure. Characterizing the dynamics of these fluids at high temperature and high pressure with nanometer spatial and picosecond temporal resolution has been very challenging. The advent of hard x-ray free electron lasers has enabled the development of novel multi-pulse ultrafast x-ray scattering techniques, such as x-ray photon correlation spectroscopy (XPCS) and x-ray pump x-ray probe (XPXP). These techniques offer new opportunities for resolving the ultrafast microscopic behavior in SCFs at unprecedented spatiotemporal resolution, unraveling the dynamics of their micro-structure. However, harnessing these capabilities requires a bespoke high-pressure and high-temperature sample system that is optimized to maximize signal intensity and address instrument-specific challenges, such as drift in beamline components, x-ray scattering background, and multi-x-ray-beam overlap. We present a pressure cell compatible with a wide range of SCFs with built-in optical access for XPCS and XPXP and discuss critical aspects of the pressure cell design, with a particular focus on the design optimization for XPCS.

Pressure-induced reversal of Peierls-like distortions elicits the polyamorphic transition in GeTe and GeSe.

While polymorphism is prevalent in crystalline solids, polyamorphism draws increasing interest in various types of amorphous solids. Recent studies suggested that supercooling of liquid phase-change materials (PCMs) induces Peierls-like distortions in their local structures, underlying their liquid-liquid transitions before vitrification. However, the mechanism of how the vitrified phases undergo a possible polyamorphic transition remains elusive. Here, using high-energy synchrotron X-rays, we can access the precise pair distribution functions under high pressure and provide clear evidence that pressure can reverse the Peierls-like distortions, eliciting a polyamorphic transition in GeTe and GeSe. Combined with simulations based on machine-learned-neural-network potential, our structural analysis reveals a high-pressure state characterized by diminished Peierls-like distortion, greater coherence length, reduced compressibility, and a narrowing bandgap. Our finding underscores the crucial role of Peierls-like distortions in amorphous octahedral systems including PCMs. These distortions can be controlled through pressure and composition, offering potentials for designing properties in PCM-based devices.

Femtosecond Reduction of Atomic Scattering Factors Triggered by Intense X-Ray Pulse.

X-ray diffraction of silicon irradiated with tightly focused femtosecond x-ray pulses (photon energy, 11.5 keV; pulse duration, 6 fs) was measured at various x-ray intensities up to 4.6×10^{19} W/cm^{2}. The measurement reveals that the diffraction intensity is highly suppressed when the x-ray intensity reaches of the order of 10^{19} W/cm^{2}. With a dedicated simulation, we confirm that the observed reduction of the diffraction intensity can be attributed to the femtosecond change in individual atomic scattering factors due to the ultrafast creation of highly ionized atoms through photoionization, Auger decay, and subsequent collisional ionization. We anticipate that this ultrafast reduction of atomic scattering factor will be a basis for new x-ray nonlinear techniques, such as pulse shortening and contrast variation x-ray scattering.

Non-thermal structural transformation of diamond driven by x-rays.

Intense x-ray pulses can cause the non-thermal structural transformation of diamond. At the SACLA XFEL facility, pump x-ray pulses triggered this phase transition, and probe x-ray pulses produced diffraction patterns. Time delays were observed from 0 to 250 fs, and the x-ray dose varied from 0.9 to 8.0 eV/atom. The intensity of the (111), (220), and (311) diffraction peaks decreased with time, indicating a disordering of the crystal lattice. From a Debye-Waller analysis, the rms atomic displacements perpendicular to the (111) planes were observed to be significantly larger than those perpendicular to the (220) or (311) planes. At a long time delay of 33 ms, graphite (002) diffraction indicates that graphitization did occur above a threshold dose of 1.2 eV/atom. These experimental results are in qualitative agreement with XTANT+ simulations using a hybrid model based on density-functional tight-binding molecular dynamics.

Spectral-brightness optimization of an X-ray free-electron laser by machine-learning-based tuning.

A machine-learning-based beam optimizer has been implemented to maximize the spectral brightness of the X-ray free-electron laser (XFEL) pulses of SACLA. A new high-resolution single-shot inline spectrometer capable of resolving features of the order of a few electronvolts was employed to measure and evaluate XFEL pulse spectra. Compared with a simple pulse-energy-based optimization, the spectral width was narrowed by half and the spectral brightness was improved by a factor of 1.7. The optimizer significantly contributes to efficient machine tuning and improvement of XFEL performance at SACLA.

Pair Distribution Function from Liquid Jet Nanoparticle Suspension using Femtosecond X-ray Pulses.

X-ray scattering data measured on femtosecond timescales at the SACLA X-ray Free Electron Laser (XFEL) facility on a suspension of HfO2 nanoparticles in a liquid jet were used for pair distribution function (PDF) analysis. Despite a non-optimal experimental setup resulting in a modest Qmax of ~8 Å-1 , a promising PDF was obtained. The main features were reproduced when comparing the XFEL PDF to a PDF obtained from data measured at the PETRA III synchrotron light source. Refining structural parameters such as unit cell dimension and particle size from the XFEL PDF provided reliable values. Although the reachable Qmax limited the obtainable information, the present results indicate that good quality PDFs can be obtained on femtosecond timescales if the experimental conditions are further optimized. The study therefore encourages a new direction in ultrafast structural science where structural features of amorphous and disordered systems can be studied.

Two-dimensional Kß-Kα fluorescence spectrum by nonlinear resonant inelastic X-ray scattering.

High sensitivity of the Kß fluorescence spectrum to electronic state is widely used to investigate spin and oxidation state of first-row transition-metal compounds. However, the complex electronic structure results in overlapping spectral features, and the interpretation may be hampered by ambiguity in resolving the spectrum into components representing different electronic states. Here, we tackle this difficulty with a nonlinear resonant inelastic X-ray scattering (RIXS) scheme, where we leverage sequential two-photon absorption to realize an inverse process of the Kß emission, and measure the successive Kα emission. The nonlinear RIXS reveals two-dimensional (2D) Kß-Kα fluorescence spectrum of copper metal, leading to better understanding of the spectral feature. We isolate 3d-related satellite peaks in the 2D spectrum, and find good agreement with our multiplet ligand field calculation. Our work not only advances the fluorescence spectroscopy, but opens the door to extend RIXS into the nonlinear regime.

Structural resolution of a small organic molecule by serial X-ray free-electron laser and electron crystallography.

Structure analysis of small crystals is important in areas ranging from synthetic organic chemistry to pharmaceutical and material sciences, as many compounds do not yield large crystals. Here we present the detailed characterization of the structure of an organic molecule, rhodamine-6G, determined at a resolution of 0.82 Å by an X-ray free-electron laser (XFEL). Direct comparison of this structure with that obtained by electron crystallography from the same sample batch of microcrystals shows that both methods can accurately distinguish the position of some of the hydrogen atoms, depending on the type of chemical bond in which they are involved. Variations in the distances measured by XFEL and electron diffraction reflect the expected differences in X-ray and electron scatterings. The reliability for atomic coordinates was found to be better with XFEL, but the electron beam showed a higher sensitivity to charges.

Towards pump-probe single-crystal XFEL refinements for small-unit-cell systems.

Serial femtosecond crystallography for small-unit-cell systems has so far seen very limited application despite obvious scientific possibilities. This is because reliable data reduction has not been available for these challenging systems. In particular, important intensity corrections such as the partiality correction critically rely on accurate determination of the crystal orientation, which is complicated by the low number of diffraction spots for small-unit-cell crystals. A data reduction pipeline capable of fully automated handling of all steps of data reduction from spot harvesting to merged structure factors has been developed. The pipeline utilizes sparse indexing based on known unit-cell parameters, seed-skewness integration, intensity corrections including an overlap-based combined Ewald sphere width and partiality correction, and a dynamically adjusted post-refinement routine. Using the pipeline, data measured on the compound K4[Pt2(P2O5H2)4]·2H2O have been successfully reduced and used to solve the structure to an R1 factor of ∼9.1%. It is expected that the pipeline will open up the field of small-unit-cell serial femtosecond crystallography experiments and allow investigations into, for example, excited states and reaction intermediate chemistry.

Seeded stimulated X-ray emission at 5.9 keV.

X-ray free-electron lasers (XFELs) provide intense pulses that can generate stimulated X-ray emission, a phenomenon that has been observed and studied in materials ranging from neon to copper. Two schemes have been employed: amplified spontaneous emission (ASE) and seeded stimulated emission (SSE), where a second color XFEL pulse provides the seed. Both phenomena are currently explored for coherent X-ray laser sources and spectroscopy. Here, we report measurements of ASE and SSE of the 5.9 keV Mn Kα1 fluorescence line from a 3.9 molar NaMnO4 solution, pumped with 7 femtosecond FWHM XFEL pulses at 6.6 keV. We observed ASE at a pump pulse intensity of 1.7 × 1019 W/cm2, consistent with earlier findings. We observed SSE at dramatically reduced pump pulse intensities down to 1.1 × 1017 W/cm2. These intensities are well within the range of many existing XFEL instruments, which supports the experimental feasibility of SSE as a tool to generate coherent X-ray pulses, spectroscopic studies of transition metal complexes, and other applications.

Delayed Onset and Directionality of X-Ray-Induced Atomic Displacements Observed on Subatomic Length Scales.

Transient structural changes of Al_{2}O_{3} on subatomic length scales following irradiation with an intense x-ray laser pulse (photon energy: 8.70 keV; pulse duration: 6 fs; fluence: 8×10^{2} J/cm^{2}) have been investigated by using an x-ray pump x-ray probe technique. The measurement reveals that aluminum and oxygen atoms remain in their original positions by ∼20 fs after the intensity maximum of the pump pulse, followed by directional atomic displacements at the fixed unit cell parameters. By comparing the experimental results and theoretical simulations, we interpret that electron excitation and relaxation triggered by the pump pulse modify the potential energy surface and drives the directional atomic displacements. Our results indicate that high-resolution x-ray structural analysis with the accuracy of 0.01 Å is feasible even with intense x-ray pulses by making the pulse duration shorter than the timescale needed to complete electron excitation and relaxation processes, which usually take up to a few tens of femtoseconds.

Single-shot spectrometer using diamond microcrystals for X-ray free-electron laser pulses.

A simple spectrometer using diffraction from diamond microcrystals has been developed to diagnose single-shot spectra of X-ray free-electron laser (XFEL) pulses. The large grain size and uniform lattice constant of the adopted crystals enable characterizing the XFEL spectrum at a resolution of a few eV from the peak shape of the powder diffraction profile. This single-shot spectrometer has been installed at beamline 3 of SACLA and is used for daily machine tuning.

Experimental evidence of tetrahedral symmetry breaking in SiO2 glass under pressure.

Bimodal behavior in the translational order of silicon's second shell in SiO2 liquid at high temperatures and high pressures has been recognized in theoretical studies, and the fraction of the S state with high tetrahedrality is considered as structural origin of the anomalous properties. However, it has not been well identified in experiment. Here we show experimental evidence of a bimodal behavior in the translational order of silicon's second shell in SiO2 glass under pressure. SiO2 glass shows tetrahedral symmetry structure with separation between the first and second shells of silicon at low pressures, which corresponds to the S state structure reported in SiO2 liquid. On the other hand, at high pressures, the silicon's second shell collapses onto the first shell, and more silicon atoms locate in the first shell. These observations indicate breaking of local tetrahedral symmetry in SiO2 glass under pressure, as well as SiO2 liquid.

Generation of intense phase-stable femtosecond hard X-ray pulse pairs.

Coherent nonlinear spectroscopies and imaging in the X-ray domain provide direct insight into the coupled motions of electrons and nuclei with resolution on the electronic length scale and timescale. The experimental realization of such techniques will strongly benefit from access to intense, coherent pairs of femtosecond X-ray pulses. We have observed phase-stable X-ray pulse pairs containing more than 3 × 107 photons at 5.9 keV (2.1 Å) with ∼1 fs duration and 2 to 5 fs separation. The highly directional pulse pairs are manifested by interference fringes in the superfluorescent and seeded stimulated manganese Kα emission induced by an X-ray free-electron laser. The fringes constitute the time-frequency X-ray analog of Young's double-slit interference, allowing for frequency domain X-ray measurements with attosecond time resolution.

Fine microstructure formation in steel under ultrafast heating and cooling.

This study evaluates phase transformation kinetics under ultrafast cooling using femtosecond X-ray diffraction for the operand measurements of the dislocation densities in Fe-0.1 mass% C-2.0 mass% Mn martensitic steel. To identify the phase transformation mechanism from austenite (γ) to martensite (α'), we used an X-ray free-electron laser and ultrafast heating and cooling techniques. A maximum cooling rate of 4.0 × 103 °C s-1 was achieved using a gas spraying technique, which is applied immediately after ultrafast heating of the sample to 1200 °C at a rate of 1.2 × 104 °C s-1. The cooling rate was sufficient to avoid bainitic transformation, and the transformation during ultrafast cooling was successfully observed. Our results showed that the cooling rate affected the dislocation density of the γ phase at high temperatures, resulting in the formation of a retained γ owing to ultrafast cooling. It was discovered that Fe-0.1 mass% C-2.0 mass% Mn martensitic steels may be in an intermediate phase during the phase transformation from face-centered-cubic γ to body-centered-cubic α' during ultrafast cooling and that lattice softening occurred in carbon steel immediately above the martensitic-transformation starting temperature. These findings will be beneficial in the study, development, and industrial utilization of functional steels.

Characterization of photoinduced normal state through charge density wave in superconducting YBa2Cu3O6.67.

The normal state of high-Tc cuprates has been considered one of the essential topics in high-temperature superconductivity research. However, compared to the high magnetic field study of it, understanding a photoinduced normal state remains elusive. Here, we explore a photoinduced normal state of YBa2Cu3O6.67 through a charge density wave (CDW) with time-resolved resonant soft x-ray scattering, as well as a high magnetic field x-ray scattering. In the nonequilibrium state where people predict a quenched superconducting state based on the previous optical spectroscopies, we experimentally observed a similar analogy to the competition between superconductivity and CDW shown in the equilibrium state. We further observe that the broken pairing states in the superconducting CuO2 plane via the optical pump lead to nucleation of three-dimensional CDW precursor correlation. Ultimately, these findings provide a critical clue that the characteristics of the photoinduced normal state show a solid resemblance to those under magnetic fields in equilibrium conditions.