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We report an updated isothermal equation of state (EoS) of molybdenum (Mo) obtained by compression in beveled and toroidal diamond-anvil cells (DACs). For an improved compression environment, we developed a copper (Cu) pressure-transmitting medium (PTM) for the toroidal diamond-anvil cell samples, as it is a soft metal compared to Mo with a well calibrated EoS. A Ne PTM was used for the conventional beveled DAC samples. The unit-cell volumes of Mo were measured to 336(1) GPa in the Cu PTM and 231.2(6) GPa in the Ne PTM at room temperature. We additionally calculated elastic stiffness and compliance constants and evaluated the uniaxial stress of Mo and Cu with pressure. A new EoS for Mo is presented from data collected in all sample environments and compared to our theoretical predictions as well as previous compression studies of Mo. The (200) lattice plane of Mo produced the lowest volumes across the pressure range of this study for all compression environments, suggesting that it is less affected by nonhydrostatic stresses in the DAC compared to the other observed diffraction planes. The presented Mo EoS is compatible with extrapolations of EoS fits of Mo in helium (He) within ∼1% at 330 GPa. Results from this work demonstrate that compressing a sample in a softer metal in the toroidal DAC can improve the compression environment and result in measured sample volumes comparable to those collected in noble-gas media at multi-megabar conditions. [ABSTRACT FROM AUTHOR]

The high-precision X-ray diffraction setup for work with diamond anvil cells (DACs) in interaction chamber 2 (IC2) of the High Energy Density instrument of the European X-ray Free-Electron Laser is described. This includes beamline optics, sample positioning and detector systems located in the multipurpose vacuum chamber. Concepts for pump-probe X-ray diffraction experiments in the DAC are described and their implementation demonstrated during the First User Community Assisted Commissioning experiment. X-ray heating and diffraction of Bi under pressure, obtained using 20 fs X-ray pulses at 17.8 keV and 2.2 MHz repetition, is illustrated through splitting of diffraction peaks, and interpreted employing finite element modeling of the sample chamber in the DAC.

Experiments accessing extreme conditions at x-ray free electron lasers (XFELs) involve rapidly evolving conditions of temperature. Here, we report time-resolved, direct measurements of temperature using spectral streaked optical pyrometry of x-ray and optical laser-heated states at the High Energy Density instrument of the European XFEL. This collection of typical experiments, coupled with numerical models, outlines the reliability, precision, and meaning of time dependent temperature measurements using optical emission at XFEL sources. Dynamic temperatures above 1500 K are measured continuously from spectrally- and temporally-resolved thermal emission at 450–850 nm, with time resolution down to 10–100 ns for 1–200 μ s streak camera windows, using single shot and integrated modes. Targets include zero-pressure foils free-standing in air and in vacuo, and high-pressure samples compressed in diamond anvil cell multi-layer targets. Radiation sources used are 20-fs hard x-ray laser pulses at 17.8 keV, in single pulses or 2.26 MHz pulse trains of up to 30 pulses, and 250-ns infrared laser single pulses. A range of further possibilities for optical measurements of visible light in x-ray laser experiments using streak optical spectroscopy are also explored, including for the study of x-ray induced optical fluorescence, which often appears as background in thermal radiation measurements. We establish several scenarios where combined emissions from multiple sources are observed and discuss their interpretation. Challenges posed by using x-ray lasers as non-invasive probes of the sample state are addressed. [ABSTRACT FROM AUTHOR]

A combined microscopic, i.e., powder and single crystal x-ray diffraction and inelastic x-ray scattering, and macroscopic, i.e., heat capacity, characterization was carried out on YB 66 between 10 and 400 K. The system crystallizes in a rather large unit cell with a lattice parameter of about 23.4 Å at room temperature. The volume thermal expansion coefficient is found to be 3.9 × 10 − 5 K − 1 at 295 K. A high Debye temperature above 1000 K was determined using several methods, 1043 (25) K , from heat capacity measurements, 1430 (59) K , from elastic constants measurements, and 1690 (198) K , from thermal expansion measurements. The thermal transport is modeled as a collection of Einstein oscillators with an Einstein temperature of 194 (2) K. The C 11 and C 44 elastic constants are above 400 and 150 GPa , respectively. The average speed of sound in YB 66 is estimated to be 9631 (380) m / s , similar to the average speed of sound in corundum, 10 000 m / s. The large volume of the unit cell and the stiffness of the crystal structure make YB 66 a promising candidate for high-resolution x-ray monochromatization applications. Candidate reflections for high-resolution resonant inelastic x-ray scattering are indicated and potential pitfalls relevant to the quality of crystals and multi-beam cases are mentioned. [ABSTRACT FROM AUTHOR]

Subtle anomalies in how the structure of metallic osmium evolves with pressure are detected using powder X-ray diffraction measurements at ultra-high static pressures; the anomaly at 440 gigapascals is attributed to an electronic transition caused by pressure-induced interactions between core electrons. Osmium at extreme pressures The platinum group element osmium is one of the most incompressible of all metals and little is known about its response to extreme compression because the required pressure regime is so difficult to reach. Leonid Dubrovinsky et al. now report powder X-ray diffraction observations of osmium at unprecedentedly high static pressures attained using the state-of-the-art micro-ball nanodiamond anvil cell technique. They detect subtle anomalies in how the structure of the metal evolves at pressures above 770 gigapascals (7.7 megabars). Advanced calculations attribute the anomalies to a type of electronic transition previously unknown for the platinum group and similar metals and caused by pressure-induced interactions between core electrons. The core-level crossing or CLC transition involves pressure-induced interactions between core electrons, leading to observable changes of the material properties. The ability to perform experiments at static pressures that affect the core electrons of even the most incompressible of elements opens up new avenues in the search for new states of matter under extreme conditions. Metallic osmium (Os) is one of the most exceptional elemental materials, having, at ambient pressure, the highest known density and one of the highest cohesive energies and melting temperatures.sup.1. It is also very incompressible.sup.2,3,4, but its high-pressure behaviour is not well understood because it has been studied.sup.2,3,4,5,6 so far only at pressures below 75 gigapascals. Here we report powder X-ray diffraction measurements on Os at multi-megabar pressures using both conventional and double-stage diamond anvil cells.sup.7, with accurate pressure determination ensured by first obtaining self-consistent equations of state of gold, platinum, and tungsten in static experiments up to 500 gigapascals. These measurements allow us to show that Os retains its hexagonal close-packed structure upon compression to over 770 gigapascals. But although its molar volume monotonically decreases with pressure, the unit cell parameter ratio of Os exhibits anomalies at approximately 150 gigapascals and 440 gigapascals. Dynamical mean-field theory calculations suggest that the former anomaly is a signature of the topological change of the Fermi surface for valence electrons. However, the anomaly at 440 gigapascals might be related to an electronic transition associated with pressure-induced interactions between core electrons. The ability to affect the core electrons under static high-pressure experimental conditions, even for incompressible metals such as Os, opens up opportunities to search for new states of matter under extreme compression., Author(s): L. Dubrovinsky [sup.1] , N. Dubrovinskaia [sup.2] , E. Bykova [sup.1] [sup.2] , M. Bykov [sup.2] , V. Prakapenka [sup.3] , C. Prescher [sup.3] , K. Glazyrin [sup.4] , [...]

The structural evolution of a liquid GaIn eutectic alloy under high temperature and high pressure is investigated by combining in situ X-ray diffraction (XRD) and ab initio molecular dynamics simulations. Both experimental and theoretical results confirm that no pressure-induced sudden structural changes are detected in the liquid state along different isotherms below 700 K. The XRD patterns indicate that the liquids at 400 and 673 K both crystallize into a tetragonal crystalline phase under high pressure, whose structure is locally face centered cubic (fcc)-like. The theoretical simulations successfully describe the atomic-scale structural evolution from disordered liquid to ordered solid phases during the isothermal compression at different temperatures, revealing a strong competition between the body-centered cubic (bcc)-like and fcc-like local atomic packings at the early stage of nucleation. The liquid can directly solidify into the bcc-like atomic packing at temperatures above 650 K, whereas this bcc-like structure becomes transient and metastable below 600 K and finally transforms into a stable fcc-like atomic packing with increasing pressure. Furthermore, a high-pressure and high-temperature "phase diagram" of the GaIn eutectic alloy is roughly constructed, providing new insight into atomic-scale disorder-to-order transition of the liquid GaIn eutectic alloy in extreme conditions. [ABSTRACT FROM AUTHOR]

The stability of high-entropy alloys (HEAs) is a key issue before their selection for industrial applications. In this study, in-situ high-pressure and high-temperature synchrotron radiation X-ray diffraction experiments have been performed on three typical HEAs Ni20Co20Fe20Mn20Cr20, Hf25Nb25Zr25Ti25, and Re25Ru25Co25Fe25 (at. %), having face-centered cubic (fcc), body-centered cubic (bcc), and hexagonal close-packed (hcp) crystal structures, respectively, up to the pressure of ~80 GPa and temperature of ~1262 K. Under the extreme conditions of the pressure and temperature, all three studied HEAs remain stable up to the maximum pressure and temperatures achieved. For these three types of studied HEAs, the pressure-dependence of the volume can be well described with the third order Birch-Murnaghan equation of state. The bulk modulus and its pressure derivative are found to be 88.3 GPa and 4 for bcc-Hf25Nb25Zr25Ti25, 193.9 GPa and 5.9 for fcc-Ni20Co20Fe20Mn20Cr20, and 304.6 GPa and 3.8 for hcp-Re25Ru25Co25Fe25 HEAs, respectively. The thermal expansion coefficient for the three studied HEAs is found to be in the order as follows: fcc-Ni20Co20Fe20Mn20Cr20>bcc-Hf25Nb25Zr25Ti25 ≈hcp-Re25Ru25Co25Fe25. [ABSTRACT FROM AUTHOR]