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.