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.

Applications of synchrotron X-ray diffraction techniques have enabled crystallographic characterization of pressure-induced phase transitions in diamond anvil cells (DACs) at megabar pressures. Accurate determination of high-pressure structures is crucial for understanding all other pressure-induced property changes. This chapter discusses current capabilities, technical challenges, and future perspectives of the multigrain techniques for high-pressure studies. Through single-crystal structure analysis of seifertite SiO2 at 129 GPa, we conclude that single-crystal structure determination and refinement is possible in general cases at megabar pressures. A nearly full convergence of the structure can be achieved applying the multigrain method, and high-quality crystallographic data can then be obtained. In addition, multigrain indexation can be applied for fast online analysis of multiphase systems during synchrotron sessions. Future development of software will certainly promote wide application of the multigrain techniques. The multigrain capabilities can be further extended to multimegabar pressures. Combination of in situ X-ray powder diffraction, multigrain indexation, and single-crystal structure determination on individual grains provides new opportunities to characterize new phases at megabar pressures and beyond.

Understanding mechanical properties and their microscopic origins is fundamental for multiple fields in condensed matter research. They are controlled by defects, dislocations, diffusion, as well as microstructures, which are not trivial to study under extreme conditions. This chapter summarizes the last 25 years of advances in high-pressure devices, X-ray measurements, and data interpretation capabilities for addressing the deformation and plasticity of materials under extreme conditions, from experiments in large-volume presses or diamond anvil cells, texture and stress analysis in powder X-ray diffraction, multigrain crystallography, to self-consistent models of materials behavior. Examples of applications are then provided in the fields of geophysics and materials science along with perspectives for studies of plastic deformation under extreme conditions in the coming years.

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.

The study of minerals under shock compression provides fundamental constraints on their response to conditions of extreme pressure, temperature, and strain rate and has applications to understanding meteorite impacts and the deep Earth. The recent development of facilities for real-time in situ X-ray diffraction studies under gun- or laser-based dynamic compression provides new capability for understanding the atomic-level structure of shocked solids. Here traditional shock pressure-density data for selected silicate minerals (garnets, tourmaline, nepheline, topaz, and spodumene) are examined through comparison of their Hugoniots with recent static compression and theoretical studies. The results provide insights into the stability of silicate structures and the possible nature of high-pressure phases under shock loading. This type of examination highlights the potential for in situ atomic-level measurements to address questions about phase transitions, transition kinetics, and structures formed under shock compression for silicate minerals.

Fundamental data on planetary materials under extreme conditions are required to establish physics-based models of planetary interiors and impact events. Dynamic compression experiments provide a means of studying material properties under the conditions of planetary interiors. Experimental shock wave studies also present a unique capability to study impact phenomena in real time, providing insight into hypervelocity collisions relevant to planetary formation and evolution. Recent experimental developments have extended the types of measurements that are possible during the nanosecond to microsecond duration of shock experiments – opening entirely new lines of inquiry. New facilities that couple dynamic compression platforms with high-flux X-ray sources have allowed for in situ X-ray diffraction under dynamic loading. Such experiments can address a range of longstanding questions, including the following: What crystallographic phases are stable under what conditions? What is their thermoelastic behavior? When do they melt or vaporize? And what phases will form on release? Answers to these questions and others will provide input for next-generation models of the structure, dynamics, and evolution of planetary interiors as well and natural impact processes.

Elastic wave velocities and densities of iron and candidate iron alloys are important properties for understanding the seismological observations of Earth’s core. Several methods have been applied to measure the elastic wave velocities of iron and iron alloys at room temperature. Recently, measurements have been extended to simultaneous high-pressure and high-temperature conditions. Birch’s law, which is the linearity between density and compressional wave velocity (VP), is applicable to the experimental results of density and VP at high pressure and room temperature. The effect of temperature on Birch’s law is discussed, and it is not negligible at temperatures greater than 1,000–2,000 K. The VP and density of hcp Fe are extrapolated to pressure and temperature conditions of the inner core. VP of hcp Fe at 330–360 GPa is higher than the inner core seismic velocity, thus suggesting that iron should be alloyed with other elements so as to reduce not only its density, but also its velocity at inner core conditions. The VP of Fe–Si, Fe–H, and Fe–C alloys is slower than that of Fe at the pressure of the inner core. If the temperature effect on Birch’s law is taken into account, Si and H can be candidates for the major light elements in the inner core, while C, O, and S may not be included or exist as minor constituents.

In honor of H. K. (David) Mao and our interactions over half a century, this chapter focuses on the techniques and application of calorimetry to high-pressure research. The chapter reviews thermodynamic concepts and calorimetric methodology. It summarizes a large body of work over many years, with emphasis on mantle mineralogy, and also discusses recent developments in a broader context, including calorimetric studies of hydrous phases, nonoxides, and nanophase materials.

This chapter reviews the tremendous progress over the past several decades in experimental research of molecular solids at high pressures. The interatomic interactions in these materials are greatly modified under pressure and generally strengthen intermolecular and weaken intramolecular bonds. This leads to the formation of structurally complex crystals and inclusion compounds at moderate pressures, where a variety of intermolecular bonds can exist. Pressing on, a great majority of molecular solids demonstrate transformations to extended (e.g., polymeric) states, which vary drastically in bonding and electronic properties. The most prominent example of such behavior is the symmetrization of hydrogen bonds in ionic ice X and metallization of hydrogen in monatomic solid. Dave Mao’s legacy in this research has been remarkable ranging from discovering and establishing the structure and properties of hydrogen clathrate hydrates at 200 MPa to investigating the structure of a mixed atomic-molecular phase IV of hydrogen at 260 GPa. New generations of scientists continue to use and build upon his technical developments, which have enabled multimegabar investigations of molecular solids, including diamond anvil cell (DAC) design, the DAC gas-loading system, and a variety of optical, electric, magnetic, and X-ray DAC probes.

Understanding mechanical properties and their microscopic origins is fundamental for multiple fields in condensed matter research. They are controlled by defects, dislocations, diffusion, as well as microstructures, which are not trivial to study under extreme conditions. This chapter summarizes the last 25 years of advances in high-pressure devices, X-ray measurements, and data interpretation capabilities for addressing the deformation and plasticity of materials under extreme conditions, from experiments in large-volume presses or diamond anvil cells, texture and stress analysis in powder X-ray diffraction, multigrain crystallography, to self-consistent models of materials behavior. Examples of applications are then provided in the fields of geophysics and materials science along with perspectives for studies of plastic deformation under extreme conditions in the coming years.

Room temperature superconductivity (RTSC) is one the most challenging and longstanding problems in solid-state physics. The Bardeen–Cooper–Schrieffer (BCS) theory (1956) explained superconductivity but could not predict high critical temperatures (Tc). Extension of the BCS theory allowed RTSC in principle; however, estimations for realistic materials gave low Tc, with the only exception being metallic hydrogen. Therefore, conventional superconductors were not considered potential RTSCs. This tendency strengthened after the experimental discovery of superconductivity in cuprates with very high Tc, up to 133 K. Later, other families of nonconventional superconductivity appeared, notably, iron-based superconductors with Tc reaching 100 K. However, the mechanism of superconductivity in these materials is still not understood, and there has been no progress for many years in increasing Tc. Unexpectedly, conventional superconductors recently showed a clear prospect to be the first RTSCs: Tc = 203 K was discovered in H3S, and then Tc = 250 K in LaH10. This breakthrough resulted from a combination of factors, including the general idea to consider hydrogen-dominant materials, the appearance of ab initio predictions of structures for searching new materials, and advances in synthesis and characterization of new superconductors at megabar pressures. There is a clear prospect to achieve higher Tc in other binary or ternary hydrides. At ambient pressures, there is also a distinct possibility for substantial superconductivity, likely in materials with strong covalent bonding.

In October of 2018, a group of scientists gathered at the Broad Branch Road campus of the Carnegie Institution for Science to celebrate 50 years of high-pressure research by Ho-Kwang “Dave” Mao at the Geophysical Laboratory. The celebration highlighted the growth of high-pressure mineral physics over the last half century, which has matured into a vibrant discipline in the physical sciences because of its intimate connections to Earth and planetary sciences, solid-state physics, and materials science. Dave’s impact in high-pressure research for over a half a century has been immense, with a history of innovation and discovery spanning from the Earth and planetary sciences to fundamental materials physics. Dave has always been an intrepid pioneer in high-pressure science, and together with his numerous colleagues and collaborators across the world he has driven the field to ever higher pressures and temperatures, guided the community in adopting and adapting a spectrum of new technologies for in situ interrogation of samples at extreme conditions, and relentlessly explored the materials that make up the deep interiors of planets. In this volume, we assemble 15 chapters from authors who have worked with, been inspired by, or mentored by Dave over his amazing career, spanning a range of subjects that covers the entire field of high-pressure mineral physics.