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.

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.

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.

Development of static high-pressure techniques over the last 50 years is reviewed from the perspective of the study of the Earth’s deep interior. Fifty years ago, laboratory high-pressure and -temperature experiments were limited to the conditions corresponding to that of near the surface of the Earth. In high-pressure mineral physics, extension of the pressure range directly made possible the study of deeper parts of the Earth, and many scientists spent great effort to improve various experimental techniques. As a result, currently it is possible to do precise X-ray experiments at the conditions corresponding to the center of the Earth: 6,400 km depth from the surface, about 360 GPa, and more than 5,000 K. Two quite different types of high-pressure apparatus are widely used these days. One is the large-volume high-pressure apparatus, and the other is the diamond anvil cell. Although the latter has the advantage of covering wider pressure and temperature conditions together with optical access to the sample, the former has the advantage of much larger sample volume, and, using internal resistance heaters, very stable and uniform high-temperature conditions can be achieved. Many different types of experimental techniques are combined with these high-pressure devices, and rich information about various properties of minerals and melts can now be obtained. Advancement of synchrotron radiation played a key role for such studies, and our knowledge about the Earth’s deep interior has increased considerably. Further efforts are continuing to extend the pressure range beyond the limits of existing high-pressure devices.

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.

Development of static high-pressure techniques over the last 50 years is reviewed from the perspective of the study of the Earth’s deep interior. Fifty years ago, laboratory high-pressure and -temperature experiments were limited to the conditions corresponding to that of near the surface of the Earth. In high-pressure mineral physics, extension of the pressure range directly made possible the study of deeper parts of the Earth, and many scientists spent great effort to improve various experimental techniques. As a result, currently it is possible to do precise X-ray experiments at the conditions corresponding to the center of the Earth: 6,400 km depth from the surface, about 360 GPa, and more than 5,000 K. Two quite different types of high-pressure apparatus are widely used these days. One is the large-volume high-pressure apparatus, and the other is the diamond anvil cell. Although the latter has the advantage of covering wider pressure and temperature conditions together with optical access to the sample, the former has the advantage of much larger sample volume, and, using internal resistance heaters, very stable and uniform high-temperature conditions can be achieved. Many different types of experimental techniques are combined with these high-pressure devices, and rich information about various properties of minerals and melts can now be obtained. Advancement of synchrotron radiation played a key role for such studies, and our knowledge about the Earth’s deep interior has increased considerably. Further efforts are continuing to extend the pressure range beyond the limits of existing high-pressure devices.

The combination of double-sided laser heating in the diamond anvil cell and detailed chemical analysis of the recovered samples is a promising approach to explore the chemistry of the Earth’s deep interior from the lower mantle to the core. Routine recovery of laser-heated samples coupled with chemical and textural characterization at the submicron scale is the key to expand knowledge of chemical interactions and melting at extreme conditions, particularly in complex systems. Recent technical developments have allowed us to investigate element partitioning and melting relations at pressures approaching the Earth’s inner-core boundary. In this chapter, we review the techniques used for recovering tiny laser-heated samples and analyzing their chemical compositions and quenched textures, while highlighting key experiments that address silicate–metal element partitioning during mantle–core differentiation, silicate melting relations with applications to early magma ocean crystallization and deep mantle melting, and melting relations in iron-alloy systems relevant to the core. The results have drastically expanded our understanding of element redistribution at deep chemical boundaries and the chemical evolution of the deep mantle and the inner core. Finally, we emphasize the need for standardized protocols to obtain consistent, reproducible results and streamlined procedures to promote good practice and increase productivity. A broad collaboration with a systematic approach would further advance the field of high-pressure geochemistry.

Waste heat – the pressure-volume area between the Rayleigh line and Hugoniot – offers a simple means of quantifying energy dissipation upon dynamic compression, confirming that (i) maximum compression on shock loading corresponds to the conditions at which all the shock energy goes into heating rather than compression; (ii) breaking a single shock into two shocks reduces heating, an effect optimized by the intermediate compression being about half the final compression; and (iii) static precompression further reduces heating upon shock loading to a given final compression. Combined static-dynamic experiments can thus maximize material compression by tuning dissipation.

Waste heat – the pressure-volume area between the Rayleigh line and Hugoniot – offers a simple means of quantifying energy dissipation upon dynamic compression, confirming that (i) maximum compression on shock loading corresponds to the conditions at which all the shock energy goes into heating rather than compression; (ii) breaking a single shock into two shocks reduces heating, an effect optimized by the intermediate compression being about half the final compression; and (iii) static precompression further reduces heating upon shock loading to a given final compression. Combined static-dynamic experiments can thus maximize material compression by tuning dissipation.

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.