Materials under high pressure: A chemical perspective

TL;DR

High pressure fundamentally alters elemental behavior, enabling novel structures and bonding, with combined theoretical and experimental approaches advancing understanding and discovery of new high-pressure phases, including potential room-temperature superconductors.

Contribution

This paper reviews how high pressure changes chemical properties and highlights recent successes in predicting and experimentally confirming new high-pressure phases and superconductors.

Findings

High pressure induces unusual oxidation states and bonding.

Crystal structure prediction effectively identifies novel high-pressure phases.

Experimental techniques confirm many predicted high-pressure structures.

Abstract

At high pressure, the typical behavior of elements dictated by the periodic table - including oxidation numbers, stoichiometries in compounds, and reactivity, to name but a few - is altered dramatically. As pressure is applied, the energetic ordering of atomic orbitals shifts, allowing core orbitals to become chemically active, atypical electron configurations to occur, and in some cases, non-atom-centered orbitals to form in the interstices of solid structures. Strange stoichiometries, structures, and bonding motifs result. Crystal structure prediction tools, not burdened by preconceived notions about structural chemistry learned at atmospheric pressure, have been applied to great success to explore phase diagrams at high pressure, identifying novel structures in diverse chemical systems. Several of these have been subsequently observed by experimental investigations, whose access to high-pressure regimes is bolstered by advances in diamond anvil cell and dynamic compression techniques. The joint efforts of experiment and theory have led to particular success in the realm of high-temperature superconductors, identifying many novel phases whose superconducting transition approaches room temperature.