Zuzana Konôpková

The phase stability, crystal structure, and melting of Nb have been examined under high pressure shock compression to 365 GPa using ultrafast x-ray diffraction measurements on two x-ray free electron laser facilities. On compression, Nb remains stable in the bcc phase up to 220 GPa, with coexistence of bcc-Nb and liquid from 249 to 298 GPa, and complete melting at 301 GPa, with melt identified by diffuse liquid diffraction. Melting initiates at higher pressure than expected based on theoretical predictions, and the data consistently excludes the presence of other suggested phases of Nb at these pressures, including and hcp, thereby resolving the long-standing structural discrepancy. Singh-type strength analysis provides experimental evidence for a change in sign of the elastic anisotropy parameter at 121 GPa, consistent with the previously reported heat-induced hardening to heat-induced softening transition in Nb.

The crystallographic structure of iron under extreme conditions is a key benchmark for state-of-the-art experimental and computational methods. Moreover, it plays a crucial role in our understanding of planetary cores, as it strongly influences the interpretation of observational data and, consequently, our knowledge of their internal structure and dynamics. However, even the crystal structure of pure solid iron under Earth's core conditions remains uncertain, with the commonly expected hexagonal close-packed phase being energetically competitive with several cubic structures. In this study, iron was compressed in a diamond anvil cell to pressures exceeding 200 GPa and dynamically probed near its melting point using MHz-frequency x-ray pulses from the European X-ray Free-Electron Laser. In the pressure range between 120 and 160 GPa, the high temporal resolution of the structural snapshots at elevated temperatures revealed the face-centered cubic phase, extending its observed stability to higher pressures than previously reported. Brief appearances of intermediate or disordered phases, including body-centered cubic (bcc) and stacking-faulted structures, were also observed. Above 230 GPa, new diffraction reflections emerged at high temperatures and are interpreted as evidence for the nucleation of a previously unreported phase, which subsequently evolved into a powderlike diffraction pattern characteristic of the bcc structure. The unprecedented time resolution of the structural data acquired during rapid heating and cooling cycles provides insights into the kinetics and mechanisms of phase transformations in iron at these extreme pressures.

This roadmap presents the state-of-the-art, current challenges and near future developments anticipated in the thriving field of warm dense matter (WDM) physics. Originating from strongly coupled plasma physics, high pressure physics and high energy density science, the WDM physics community has recently taken a giant leap forward. This is due to spectacular developments in laser technology, diagnostic capabilities, and computer simulation techniques. Only in the last decade has it become possible to perform accurate enough simulations & experiments to truly verify theoretical results as well as to reliably design experiments based on predictions. Consequently, this roadmap discusses recent developments of and contemporary challenges for theoretical methods and experimental techniques needed to describe, create and diagnose WDM. A large part of this roadmap is dedicated to specific WDM systems and applications in astrophysics, inertial confinement fusion and novel material synthesis.

Ultrafast, high-power lasers operating in the near-infrared (NIR) region are key to accelerating ions to extremely high energies. By changing the laser wavelength from the NIR region to the hard X-ray range, the photon energy increases more than 10 000 times. The interaction mechanisms and, consequently, radiation attenuation lengths differ significantly between these two spectral ranges. Here we report the use of an X-ray free-electron laser (European XFEL, Germany) delivering 9.3 keV photons in 25 fs pulses and a total energy of 0.35 mJ on a solid target. Electrons and ions escaping from an irradiated 3 μm Cu foil into vacuum were investigated by a time-of-flight technique using windowless electron multipliers that enable the measurement of very weak currents. A model based on a shifted Maxwell–Boltzmann velocity distribution of species was used to analyze the detector signals. The method used made it possible to determine the temperatures of hot electrons and protons, their center-of-mass energy, the charge states of the isotopes 63Cu and 65Cu, and the magnitude of the voltage arising in the double layer that accelerated them, and to estimate the repetition frequency of their cascade emission from the plasma. Computer simulations revealed the evolution of the electron density and temperature, the ion charge state distribution, and the time scales of processes occurring in the bulk of irradiated matter. Good correlation of theoretical and experimental results over the range of high-energy-density states demonstrates the capability to provide critical data to develop plasma models in the warm dense matter regime.

We demonstrate a grazing-incidence X-ray platform that simultaneously records time-resolved grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence X-ray diffraction (GID) from a femtosecond-laser-irradiated gold film above the melting threshold, with picosecond resolution using an X-ray free-electron laser (XFEL). By tuning the X-ray incidence angle, the probe depth is set to tens of nanometres, enabling depth-selective sensitivity to near-surface dynamics. GISAXS resolves ultrafast changes in surface nanomorphology (correlation length, roughness), while GID quantifies subsurface lattice compression, grain orientation, melting and recrystallization. The approach overcomes photon-flux limitations of synchrotron grazing-incidence geometries and provides stringent, time-resolved benchmarks for complex theoretical models of ultrafast laser–matter interaction and warm dense matter. Looking ahead, the same depth-selective methodology is well suited to inertial confinement fusion (ICF): it can visualize buried-interface perturbations and interfacial thermal resistance on micron to sub-micron scales that affect instability seeding and burn propagation.

The α (bcc)- ɛ (hcp) phase transition in polycrystalline Fe has been investigated under fast quasihydrostatic compression using piezoelectric actuator driven dynamic diamond anvil cells. X-ray diffraction data from Fe under intermediate strain rate compression were collected at megahertz repetition rates. Our results demonstrate, for the first time, the full evolution of the α − ɛ phase transition in powder and foil samples at strain rates of ∼ 10 2 s − 1 . Under fast compression, the majority of the α phase transforms to ɛ -Fe in a timescale of tens of microseconds, with a corresponding coexistence pressure of ∼ 2 GPa . The observed onset phase transition pressure of 13.4–14.4 GPa is in good agreement with the results of previous quasihydrostatic compression studies, suggesting that the transition is not influenced by compression rates up to ∼ 80 TPa s − 1 .

The development of pulsed intense x-ray sources, such as free electron laser, offers new avenues for high pressure experiments. Here, we study the feasibility and metrology of x-ray heating in diamond anvil cells at the European x-ray free electron laser. This method enables one to volumetrically heat the sample while inhibiting chemical migration and probing the crystallographic structure of the sample throughout the heating with a high repetition rate. We focus our study on iron, whose phase diagram is well established up to 100 GPa, to explore the possibilities and limitations of this technique. We volumetrically heat iron samples at starting pressures ranging from 10 to 138 GPa, using the x-ray beam pulsed at 4.5 MHz in a serial pump-and-probe experimental design. Experimental challenges arise from temperature gradients within the sample, changes in temperature at the 100 ns timescale, the difficulty of direct temperature estimates, the effect of thermal pressure, and the presence of metastable crystallites due to rapid cycles of heating and cooling. Hence, we develop a multi-crystal-like data processing method that allows us to account for sample heterogeneity in probed conditions. We then calibrate our measurements using known physical properties of iron under pressure. Thermal pressure in our experiments increases from 4% of the isochoric prediction at 10 GPa to 23% at 138 GPa, and we show that our data are in agreement with most previous observations of iron in this pressure range. The method can now be implemented at higher pressures and temperatures and on materials with unknown phase diagrams.

To address new applications in the 18–30 keV photon energy range at the European XFEL, where silicon sensors lose quantum efficiency, the AGIPD Collaboration has developed an AGIPD detector prototype with high-Z sensor materials. An electron-collecting version of the chip (ecAGIPD) was designed to leverage from the higher mobility and longer lifetime of electrons with respect to holes in the candidate materials: chromium-doped gallium arsenide (GaAs:Cr) and high-flux cadmium zinc telluride (CdZnTe). This work reports on the characterization of GaAs and high-flux CdZnTe ecAGIPD prototypes at the HED instrument at the European XFEL. Their time response, linearity and performance at 2.2 and 4.5 MHz frame rates were evaluated. Preliminary results demonstrate good linearity of both materials up to 1.6e+03 15 keV photons/mm ² /pulse, and a residual after-pulse signal corresponding to less than one photon on CdZnTe, up to an estimated flux of 1.2e+05 24 keV photons/mm ² /pulse.

We present a model of x-ray thermal diffuse scattering (TDS) from a cubic polycrystal with an arbitrary crystallographic texture, based on the classic approach of Warren [B. E. Warren, Acta Crystallogr. 6, 803 (1953)]. We compare the predictions of our model with femtosecond x-ray diffraction patterns gathered from ambient and dynamically compressed rolled copper foils obtained at the High Energy Density instrument of the European X-Ray Free-Electron Laser facility and find that the texture-aware TDS model yields more accurate results than does the conventional powder model owed to Warren. Nevertheless, we further show: with sufficient angular detector coverage, the TDS signal is largely unchanged by sample orientation and in all cases strongly resembles the signal from a perfectly random powder; shot-to-shot fluctuations in the TDS signal resulting from grain-sampling statistics are at the percent level, in stark contrast to the fluctuations in the Bragg-peak intensities (which are over an order of magnitude greater); and TDS is largely unchanged even following texture evolution caused by compression-induced plastic deformation. We conclude that TDS is robust against texture variation, making it a flexible temperature diagnostic applicable just as well to off-the-shelf commercial foils as to ideal powders.

High-pressure experiments allow us to induce and control chemical reactions or synthesize materials of unique properties. Furthermore, they are our primary means to replicate the conditions of geological processes and thus of studying, measuring, and understanding compositions and conditions in planetary interiors or asteroid impacts and subduction, etc. In all those experiments, time is a critical factor, as the compression rate or strain rate defines and alters the sample environment. Until recently, most experiments have been performed with either quasistatic strain rates, ≤ 10 − 3 s − 1 , in diamond anvil cell and large volume press apparatus, or at very high strain rates, ≥ 10 5 s − 1 , using gas gun, laser shock, and ramp compression. The work presented here focuses on intermediate compression rates applied to hcp Zn, using the dynamic diamond anvil cell. The evolution of the lattice parameters depends on the choice of the pressure transmitting medium. Nevertheless, at nonhydrostatic conditions we observe a significant change in the evolution of the lattice parameter when compressed at strain rates of 2 × 10 − 2 s − 1 or 2.7 × 10 2 s − 1 . At 2.7 × 10 2 s − 1 , the ratio does not correlate with the results of slow nonhydrostatic experiments, but its trend resembles closer with hydrostatic, quasistatic data. The deviation indicates that strain rates of 2.7 × 10 2 s − 1 interfere with the otherwise predominant deformation mechanism at quasistatic conditions.