Crystal chemistry at high pressure

The chemistry we are taught in school, and we experience in our daily existence occurs at 1atm. However, pressure spans an astounding 62 orders of magnitude in the Universe—in going from the void of interstellar space to the crushing conditions at the center of a neutron star. The way in which pressure affects chemistry is important for Earth and planetary sciences, materials science, in understanding the extreme conditions experienced in nuclear explosions, and it may be key in addressing the future energy needs of our society. In this article we outline how the often neglected pressure variable affects chemistry, beginning from the way in which atomic energy levels are altered. This lays the foundation for understanding the unique crystal and electronic structures that emerge when matter is squeezed, as well as pressure’s effect on chemical reactivity. Finally, we give an overview of the main concepts behind conventional, or phonon-mediated, superconductivity, and describe how the pressure variable may be key in discovering and designing light-element based materials whose superconducting critical temperatures approach room temperature. We discuss some of the main families of superconducting hydrides that have been predicted computationally, and the experimental successes in this exciting and rapidly developing field.