Ten years ago, Dave Mao, director of Energy Frontier Research in Extreme Environments (EFree), a Department of Energy (DOE) energy frontier, recognized the importance of neutron science for energy research. The subsequent establishment of a neutron group within EFree lead to the formation of an Instrument Development Team for SNAP, the dedicated high-pressure beamline at the Spallation Neutron Source at Oak Ridge National Laboratory in Tennessee. The core concept was to develop novel high-pressure techniques to expand the pressure range for neutron diffraction. A quite ambitious goal was set to reach half megabar levels (50 GPa), which at the time was considered extremely challenging. Here we will give a brief overview of the developments during the last decade in this novel area of research. An important factor was that during this period multicarat diamond anvils have become available grown by chemical vapor deposition (CVD), making research in this pressure range and beyond rather routine. This chapter shows the latest developments in large anvil and anvil support designs, compact multiple ton diamond cells, and new neutron methodologies. Achievements are illustrated with some examples of high-quality neutron diffraction patterns collected on sample sizes much small than conventional sizes.

Accelerator-based hard X-ray sources (storage-ring synchrotron radiation, and X-ray free electron laser, or FEL) provide X-ray beams with high energy, high brilliance, short tens-of-picosecond-to-femtosecond pulses, and high coherence that are well suited for high-pressure studies. Developments in high-pressure technology, advanced X-ray optics and detectors, and synergies with theoretical computations have helped drive the rapid growth of high-pressure research using synchrotron and FEL X-rays. In this chapter, we present a brief review of the research field from a historical perspective, illustrated by selected aspects on research using the diamond anvil cell. We then highlight a few of the active areas in high-pressure X-ray research, including ultrahigh-pressure generation, amorphous materials at high pressure, phase transition kinetics, and materials metastability. Finally, an outlook on future directions and opportunities with the upgrades in both synchrotron and FEL facilities worldwide is presented.

Our ability to determine the density (specific volume) as a function of pressure and temperature has drastically improved in the last several decades, with the combination of synchrotron X-ray diffraction and high-pressure techniques such as laser-heated diamond-anvil cell and large-volume multi-anvil press. The improvements are in both pressure–temperature range and data quality, and obtaining high-resolution 2D angle-dispersive diffraction data at over a megabar pressure and above 2,500 K is now routine. In parallel, dynamic compression techniques, such as laser-driven shock wave and magnetically accelerated flyer plate-impact experiments, have provided new ways to measure density at extreme conditions. The combination of static and dynamic compression data allows us to examine internal consistency in pressure determination and establish reliable pressure scales. Internally consistent pressure scales for several pressure standards are emerging through extensive comparison of compression data over a large pressure range and simultaneous measurements of elasticity and density. A concerted effort is needed to further expand and improve measurements under simultaneous high pressure and temperature, particularly at temperatures above 2,500 K, in order to accurately model the thermal pressure. To decipher the compositions of the Earth’s interior based on density observations from seismology requires high accuracy in measuring the subtle compositional effects on the density of mantle and core materials. For a universal understanding of the thermal equations of state of solids, the emphasis should be on reconciling the static and dynamic data of well-studied materials that have substantial overlap in pressure–temperature ranges.

Accelerator-based hard X-ray sources (storage-ring synchrotron radiation, and X-ray free electron laser, or FEL) provide X-ray beams with high energy, high brilliance, short tens-of-picosecond-to-femtosecond pulses, and high coherence that are well suited for high-pressure studies. Developments in high-pressure technology, advanced X-ray optics and detectors, and synergies with theoretical computations have helped drive the rapid growth of high-pressure research using synchrotron and FEL X-rays. In this chapter, we present a brief review of the research field from a historical perspective, illustrated by selected aspects on research using the diamond anvil cell. We then highlight a few of the active areas in high-pressure X-ray research, including ultrahigh-pressure generation, amorphous materials at high pressure, phase transition kinetics, and materials metastability. Finally, an outlook on future directions and opportunities with the upgrades in both synchrotron and FEL facilities worldwide is presented.