E. F. O’bAnnon

The melting temperature of elemental bismuth under high pressure has been measured to 55 GPa using synchrotron x-ray phase-contrast imaging in the laser-heated diamond anvil cell. Imaging of solid-liquid interface formation, combined with radiometric temperature and x-ray diffraction measurements, reveals a pronounced reduction in melting boundary slope in Bi-V with pressure. The unusually steep initial slope is attributed to low configurational entropy of melting, arising from structural ordering and coordination matching in the cool liquid, while slope reduction is driven by entropy increase correlated with significant liquid structure changes with rising pressure and temperature. The data rule out kinetic effects on melting in shock compression experiments and demonstrate the need for improved theoretical phase diagrams.

We have developed a new ceramic–graphite composite heater for use in the diamond anvil cell. These new heaters have shown the capability to rapidly heat samples at average rates as high as 30 K/s and have been tested up to a maximum sample temperature of 1100 K. We present four scientific case studies in which we test the functionality of the heaters and verify the accuracy of the thermocouple readings by benchmarking to calibrated material responses and to known phase transitions in select materials. The first case study employs time resolved fluorescence measurements of Sm:YAG and ruby, using the Sm:YAG fluorescence to extract the pressure of the cell to determine the temperature based on the ruby measurement. The second case study is a time resolved Raman study of sulfur at high pressure and temperature where we reproduce the known solid–liquid and liquid–liquid transitions. Additionally, in this portion of the study, we also verify the high temperature transition to a trigonal crystal structure. In the third case study, we performed stepwise heating of Sn up to 1100 K and accessed the liquid phase at two different pressures. For the fourth and final case study, we performed time resolved optical microscopy measurements on H2O with the goal of dynamically compressing from liquid H2O into ice-VII at high temperature.

The volume compression of body-centered cubic tantalum has been measured to 433 GPa in the standard bevel and toroidal diamond anvil cell (DAC) using synchrotron-based angle-dispersive powder x-ray diffraction. Six experiments were carried out achieving a maximum pressure of 433(6) GPa. Pressures were estimated using an equation of state (EoS) of Cu or Bi for all experiments. We additionally calculated the elastic constants of Ta and Bi and used our previously calculated values for Cu to inform the stress state of the sample and an EoS of Ta for comparison to our experimental data. Our experimental pressure–volume data for Ta was fit with a fourth-order Vinet EoS, resulting in parameters K0 = 192(5) GPa, η = 6.4(6), β = −35(5), and Ψ = 141(13). This Ta EoS is consistent within uncertainty with the ultrasonically derived ambient isothermal bulk modulus, with existing DAC data up to 100 GPa, and with Ta ramp EoS measurements that exceed the pressures of the current study.

With the increasing number of 100 s GPa experiments in the diamond anvil cell (DAC), improved accuracy in secondary pressure calibrations to extreme pressures is essential. The rhenium equation of state has been proposed as a pressure calibrant via x-ray diffraction with potentially broad applications as it is commonly used as a gasket material in DAC experiments. In this work, we conducted Raman spectroscopy experiments on rhenium in the DAC and report the pressure shift of the E2g mode, a refined high-pressure C44 and mode-Grüneisen parameter above 200 GPa. We used flat, beveled, and toroidal diamond anvils under quasi-hydrostatic and non-hydrostatic conditions. By measuring the E2g mode from the culet edge to the center, we analyzed pressure distribution based on culet type and distance from the anvil center. The shift in the E2g mode can be expressed as a function of pressure, and diamond edge measurements appear reliable across all anvil types. Comparing the center and edge pressures reveals anvil cupping, offering insights into predicting or preventing anvil failure during materials properties measurements at extreme conditions.

The high pressure equation of state for the W 0.75 R e 0.25 alloy is experimentally determined up to 183 GPa with synchrotron angle-dispersive powder x-ray diffraction in the diamond-anvil cell and to ∼925 GPa with density-functional theory. W-Re alloys are used in many industrial high-temperature applications and as a confining gasket material in high-pressure diamond-anvil cell research. The inclusion of 25 wt. % Re achieves the highest performance in terms of strength and ductility while also maintaining the body-centered-cubic (bcc) crystal structure, yet to date there has been no investigation into its elastic behavior at high pressure. We present the experimentally and theoretically determined volumetric and elastic pressure response and systematically compare these results to other W-Re alloys, finding that the bulk modulus of W-Re alloys varies nonlinearly with Re content and W 0.75 Re 0.25 becomes less incompressible than W at 85 GPa.

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.

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 μs 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.