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In the laboratory study of extreme conditions of temperature and density, the exposure of matter to high intensity radiation sources has been of central importance. Here, we interrogate the performance of multi-layered targets in experiments involving high intensity, hard x-ray irradiation, motivated by the advent of extremely high brightness hard x-ray sources, such as free electron lasers and 4th-generation synchrotron facilities. Intense hard x-ray beams can deliver significant energy in targets having thick x-ray transparent layers (tampers) around samples of interest for the study of novel states of matter and materials' dynamics. Heated-state lifetimes in such targets can approach the microsecond level, regardless of radiation pulse duration, enabling the exploration of conditions of local thermal and thermodynamic equilibrium at extreme temperature in solid density matter. The thermal and mechanical responses of such thick layered targets following x-ray heating, including hydrodynamic relaxation and heat flow on picosecond to millisecond timescales, are modeled using radiation hydrocode simulation, finite element analysis, and thermodynamic calculations. Assessing the potential for target survival over one or more exposures and resistance to damage arising from heating and resulting mechanical stresses, this study doubles as an investigation into the performance of diamond anvil high pressure cells under high x-ray fluences. Long used in conjunction with synchrotron x-ray radiation and high power optical lasers, the strong confinement afforded by such cells suggests novel applications at emerging high intensity x-ray facilities and new routes to studying thermodynamic equilibrium states of warm, very dense matter. [ABSTRACT FROM AUTHOR]

We have performed in situ synchrotron X-ray diffraction experiments on TaC0.99 compressed in a diamond anvil cell along 3 isothermal paths to maximum pressure (P)-temperature (T) conditions of 38.8 GPa at 1073 K. By combining measurements performed in axial diffraction geometry at 296 K and in radial geometry at 673 K and 1073 K, we place constraints on the pressure-volume-temperature (P-V-T) equation of state of TaC in a wide range of conditions. A fit of the Birch-Murnaghan equation to the measurements performed in axial geometry at ambient temperature yields a value of the isothermal bulk modulus at ambient conditions K T 0 = 305 ± 5 (1 σ) GPa and its pressure derivative (∂ K T / ∂ P) T 0 = 6.1 ± 0.5. The fit of the Birch-Murnaghan-Debye model to our complete P-V-T dataset allows us to constrain the Grüneisen parameter at ambient pressure γ 0 = V (∂ P / ∂ E) V 0 to the value of 1.2 ± 0.1. [ABSTRACT FROM AUTHOR]

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]

As-cast amorphous AsSe (a-AsSe) has been characterized by in-situ high pressure XRD and Raman spectroscopy up to the pressure of ~30GPa using diamond anvil cell together with ab-initio molecular dynamics simulations. A gradual densification has been observed under compression along with the breakdown of intermediate range ordering at ~16GPa. The whole transformation process can be divided into three relatively distinct pressure regimes from 1 bar to 7GPa, from 7 to 16GPa, and beyond 16GPa. Our XRD results together with Raman spectroscopic studies confirm that in the a-AsSe pressure tuning results in network transformations only, without sudden jump in the density. The results obtained by high pressure ab-initio molecular dynamics simulations demonstrate the variations in the local structures associated with the experimentally observed transformations. The amorphous-to-amorphous network transformation is found to be reversible upon decompression. [ABSTRACT FROM AUTHOR]

We performed high-pressure synchrotron x-ray diffraction experiments on nanocrystalline (nc-) MgO compressed both under quasi-hydrostatic and non-hydrostatic conditions in a diamond-anvil cell. Data obtained under hydrostatic conditions show that nc-MgO (average crystallite size of 20 nm) is 8-9% more compressible than 'bulk' MgO. Analysis of our results collected under non-hydrostatic conditions yields a bulk modulus that is about 27% larger than the one derived from the quasi-hydrostatic compression experiments. Thus, the apparent bulk modulus strongly depends on the experimental stress conditions. [ABSTRACT FROM AUTHOR]