The development of diamond-anvil cells together with the improvement of the characterization techniques in large facilities has led to the expansion of high-pressure (HP) research. The application of HP has given rise to many important breakthroughs during the past decade because it can radically change the physical and chemical properties of materials yielding unexpected modifications. For instance, HP has allowed the discovery of new materials or new phases of known materials with unique properties, such as high T_Csuperconductors, Mott insulators, half-metals, etc.

Here, we present the study of several striking materials by two complementary techniques: x-ray absorption spectroscopy (XAS) and x-ray diffraction (XRD), both carried out in the European Synchrotron Radiation Facility (ESRF). Firstly, the iridium (Ir) metal has been investigated up to 1.4 Mbar in order to discover experimentally, for the first time, a new electronic transition predicted in the majority of 5d transition metals [1]. This new transition is known as Core Level Crossing and it involves the overlap between the 4f/5p core levels affecting the 5d valence orbitals yielding a change in the chemical bonds. The structural stability (Fig.1a) and the electronic properties of Ir metal were studied by XRD at ID15B beamline and XAS at BM23 beamline, respectively. Secondly, we have stablished a physical model to explain the pressure-induced modifications in the electronic structure of europium (Eu) monochalcogenides, EuX (X = O, S, Se, Te). All of them exhibit optical, electric and magnetic anomalies around 14 GPa [2] but the reason behind them had never been unveiled so far. We have studied one of these compounds, the EuS, by XAS up to 35 GPa at BM23 beamline (Fig. 1b). Finally, the copper(II) oxide, CuO, itself has seen renewed interest due to the discovery of multiferroicity (MF) at relatively high temperature T_N = 230 K and ambient pressure [3]. However, such a discovery is not free of controversy since different researchers have obtained contradictory results [4]. We have carried out a XAS experiment up to 18 GPa, at BM23 beamline, to analyse the static and dynamics contribution of the local environment around Cu atoms (Fig. 1c) shedding some light on this scientific problem.

Despite being the subject of numerous shock compression studies, the behavior of silicon under dynamic loading is vigorously debated [1-4]. The few studies that combine shock compression and X-ray diffraction have exclusively focused on "normal" X-ray geometry whereby X-rays are collected along the shock propagation direction, consequently sampling numerous strain states at once, and hence greatly complicating both phase identification and studies of phase transition kinetics.[5] Here, we present a novel setup to perform in situ X-ray diffraction studies perpendicular to the shock propagation direction at the Matter in Extreme Conditions end station at Linac Coherent Light Source, SLAC. Combining the extremely bright, micro-focused X-ray beam available at the LCLS with a nanosecond laser driver, we unambiguously characterize of the complex multi-wave shock response in silicon for the first time. We further combine this platform for performing simultaneous imaging with X-ray diffraction from shock compressed germanium, revealing its behaviour following shock compression. We note the transverse geometry is significantly more sensitive to the onset of both solid-solid and solid-liquid phase transformations in materials which exhibit complex multi-wave behaviour, and compare and constrast the behaviour of Si and Ge.

[1] Graham et al., JPCS, 27, 9 (1966)

[2] Turneaure & Gupta, APL, 90, 051905 (2007)

[3] Colburn et al., JAP, 43, 5007 (1972)

[4] Gust & Royce, JAP, 42, 1897 (1971)

[5] Turneaure et al., PRL,121, 135701 (2018)

We have developed and tested a new internally heated diamond anvil cell (DAC) as reported in a recent paper published in Review of Scientific Instruments [1]. The system includes a portable vacuum chamber and was designed for routine performance of x-ray and optical experiments. We have adopted a self-heating W/Re gasket design allowing for both sample confinement and heating. This solution proved to be very efficient to improve heating and cooling rates in a temperature regime up to 1500 K. The system has been widely tested and calibrated under high-temperature conditions. The temperature distribution was measured by in situ optical measurements and resulted to be uniform within the typical uncertainty of the measurements (5% at 1000 K). XAS (x-ray absorption spectroscopy) of pure Ge at 3.5 GPa were easily obtained in the 300 K–1300 K range, studying the melting transition and nucleation to the crystal phase. An original XAS-based dynamical temperature calibration procedure was developed and used to monitor the sample and diamond temperatures, indicating that heating and cooling rates in the 100 K/s range can be easily achieved using this device.

The EMA (Extreme condition Methods of Analysis) is one of the first Sirius beamlines, the 4th generation Brazilian synchrotron source. Currently under commissioning, its focus is on merging extreme thermodynamic conditions with a solid characterization platform based on spectroscopy and scattering techniques. For this, it has an undulator source and optics based on a high-dynamic double-crystal monochromator (HD-DCM) [1], 1/4 wave plates (double phase retarder), and KB-mirrors, which can provide X-ray beam sizes as small as ~ 1 x 0.5 μm² and ~ 100 nm², respectively, for two experimental stations (Microfocus and Nanofocus hutches).

Here is shown the current developments for the XRD experiments performed on both the “multipurpose setup” (@ 45 m from the source) and the 6‑circle diffractometer (@ 55 m) [2]. The former relies on a hexapod for sample positioning and angular scans and offers the most extreme temperature (0.5 – 5000 K) and pressure (~ 600 GPa) within the Microfocus hutch, allied with a 1 T magnet. The latter setup will work with tunable beam sizes ranging from ~ 13 x 3 up to ~ 300 x 300 μm² and positioning systems on top of its inner circle that delivers up to 66 mm of free working distance, expanding the possibilities for third-party sample mountings and environments, such as an available uniaxial strain cell. For this setup, the significant range in temperature (5 – 300 K), pressure (~300 GPa), and magnetic field (6T) add flexibility to the already versatile 4S+2D diffractometer. Additionally, opportunities for a broad sort of techniques supported at EMA will be discussed for powder and single-crystalline samples.

Fast EXAFS measurement in piezo-driven single-crystal monochromatization scheme

At the “Langmuir” station of the Kurchatov Synchrotron-Neutron Research Complex, a single-crystal monochromator based on adaptive bending X-ray acoustic element [1] was implemented for X-ray beam energy fast tuning and for rapid recording K-edge absorption spectra (XANES-spectrum) of Bromine in NaBr powder sample.

To control beam parameters and record the absorption spectrum, Si single-crustal monochromator, driven by ultrasonic vibrations excitation in piezo-actuator, and monitoring system were used. Diffracted synchrotron beam was collimated by slits and recorded using a scintillation detector, connected with multi-channel analyzer. X-ray acoustic element was excited via the inverse piezoelectric effect by applying a AC electronic signal with first harmonic resonance frequency f_rez = 239 Hz. During the experiments, the beam intensity was recorded in relation to control signal phase, further converted into an absorption spectrum.

After data processing the results it was established that the position of absorption edge and the first coordination sphere radius coincided for X-ray acoustic and traditional mechanical scan. Achieved energy scan range was 13.25–13.65 keV (400 eV). Maximum time resolution available using the x-ray acoustic method is 2.1 ms, and actual time required to record qualitative spectrum, achieved in this experiment, was about 30 seconds and can be reduced by using detector with a higher dynamic range and counting rate, as well as optimizing X-ray optical scheme.

The developed scheme is promising for QEXAFS methods implementation, useful for chemical reactions kinetics study, for example, the Belousov-Zhabotinsky self-oscillation reaction [2], as well as the deformation processes kinetics research under external influences.

This work was partially supported by RFBR grants No. 18-32-20108 mol_a_ved, as well as the Council on Grants of the President of the Russian Federation МК-2451.2018.2.

1. A.E. Blagov, A.S. Bykov et al. PTE, 2016, No. 5, p. 109

2. M Hagelstein, T Liu et al. // J. of Physics: Conference Series 430 (2013) 012123

Skutterudite-type compounds based on □Co₄Sb₁₂ pnictide are promising for thermoelectric application due to their good Seebeck values and high carrier mobility. Filling the 8a voids (in the cubic space group Im3̅) with different elements (alkali, alkali earth, and rare earth) helps to reduce the thermal conductivity and thus increases the thermoelectric performance. A systematic characterization by synchrotron X-ray powder diffraction of different M-filled Co₄Sb₁₂ (M = K, Sr, La, Ce, and Yb) skutterudites was carried out under high pressure in the range ∼0–12 GPa. The isothermal equations of state (EOS) were obtained in this pressure range and the Bulk moduli (B₀) were calculated for all the filled skutterudites, yielding unexpected results. A lattice expansion due to the filler elements fails in the description of the Bulk moduli. Topochemical studies of the filler site environment exhibited a slight disturbance and an increased ionic character when the filler is incorporated. The mechanical properties by means of Bulk moduli resulted in being sensitive to the presence of filler atoms inside the skutterudite voids, being affected by the covalent/ionic exchange of the Co–Sb and Sb–Sb bonds.

The extreme densities of matter relevant to most exoplanets are not reachable by static compression, i.e., in diamond anvil cell (DAC), or by single shock compression techniques. Multiple shocks generated by tailored laser pulses allow reaching higher densities, but the thermodynamic state of the system is not easy to measure. Instead of using the multi-shock compression technique, we can send a laser-induced shock wave through a sample that is pre-compressed at high static pressures inside a DAC. The equation-of-state of the system is then directly measured through the Rankine-Hugoniot equations from the shock and particle velocities, and temperature can be measured independently with pyrometry. Several experiments demonstrated the combination of these two techniques [1-5] at kJ laser facilities and documented material properties at unprecedented conditions.

We introduce a new design of a shock diamond anvil cell (SDAC) for sub-kJ laser-driven dynamic compression experiments at X-ray sources. We designed a system of two thin diamond anvils, one of which is perforated. The perforation is envisioned to allow shock waves created by low/moderate energy lasers to propagate through the sample. Being developed to be usable by any user community at the High Energy Density (HED) instrument at European-XFEL, or other large-scale facilities around the world, the unique design of the SDAC will make it possible to reach higher density states of matter in dynamic compression experiments and probe previously unexplored regions of the pressure-temperature-density phase diagram, combined with x-ray techniques at XFEL sources. We will present technical details and first results of the pre-compression pressures achieved using SDAC along with hydrodynamic simulation results of dense Krypton, among other samples, laser-shocked at different initial densities.

[1] Loubeyre, P. et al (2003). High Pressure Research. 24, 1, 25-31

[2] Eggert, J. et al (2008). Phys. Rev. Lett. 100, 124503

[3] Celliers, P. M. et al (2010). Phys. Rev. Lett. 104, 184503

[4] Loubeyre, P. et al (2012). Phys. Rev. B. 86, 144115

[5] Millot, M. et al (2018). Nat. Phys. 14, 297-302

Part of this work was prepared by LLNL under Contract DE-AC52-07NA27344 and supported by LDRD 19-ERD-031.