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states of matter at extreme conditions Most materials in the universe experience extreme conditions of high pressure and high temperature, as they exist deep in stars and planets (Figure 1). The extreme conditions span more than 10 orders of magnitude. The compression energy (based on low-Z molecules such as nitrogen) reaches 1 meV at 1 kbar (0.1 GPa), several eV at 1 Mbar (100 GPa), and 1 keV at 1 Gbar (100 TPa). As a result, materials undergo significant changes in their bonding, structures, and properties. The pressure at the bottom of the ocean is about 1 kbar (0.1 GPa), where the compression energy (∼1 meV) rivals the hydrogen bonding energy and drives the formation of water clathrates with the capture of small molecules such as methane—the most abundant energy source in the Earth.1 At 50–100 GPa, the compression energy can exceed several eV, competitive with the chemical bond energy in simple molecules. Under such conditions, the low-Z elemental solids in the first and second rows of the periodic table, while existing as simple molecular or elemental solids at ambient conditions, form unusual dense covalent or ionic network structures such as diamond,2 symmetric ice,3 and metallic hydrogen.4 Above 1 TPa, the atoms are brought so close that even valence electrons can assemble together as quasiparticles,5 which fill the interstitial sites of core electron nuclei and form novel electrides (electron-trapped solid materials).6 At higher pressures of 10–100 TPa, the compression energy can reach the

core electron energy (keV), triggering nuclear chemistry in dense solids. Recent developments of high-pressure technologies make it possible to generate extreme conditions in the laboratory. These include large volume presses and diamond anvil cells (DACs), which can generate static pressures up to and beyond 300 GPa. Dynamic pressure capabilities such as two-stage gas guns, the Z pulse-power machine at Sandia National Laboratories,7 and high-power lasers extend the static pressure to 1 TPa and higher.8 Combining these high-pressure techniques with advanced light sources such as third-generation synchrotron x-rays, x-ray free-electron lasers (XFELs), and spallation neutron sources, as well as first-principles theoretical developments using large computational platforms, provides opportunities to discover new materials and exploit new chemistry under extreme conditions.

Predictive solid-state chemistry The pressure-induced chemistry occurring at the compression energy of constituent chemical bonds offers a new way of controlling solid-state reactions, as the energies of direct lattice compression are comparable to those of chemical bonds. This is unlike solid-state reactions of inorganic materials at near ambient conditions, which are largely reliant upon the kinetics of limited mass diffusion and the unpredictability of defect mobility for their investigation. Modern

Choong-Shik Yoo, Department of Chemistry, Institute for Shock Physics, and Materials Science and Engineering, Washington State University, USA; [email protected]