Matter in Extreme Environments: Novel Chemistry under Pressure

Thanks to the developments of high-pressure techniques and quantum mechanics based crystal structure prediction methods, numerous novel compounds with atypical compositions have been obtained or predicted in the past decade. Differing from conventional solid materials, many of these new compounds consist of various homonuclear chemical species such as dimers, trimers, pentagonal and heptagonal rings, polymer chains, atomic layers, and three-dimensional networks, unexpectedly telling a story of rich chemistry under pressure. More strikingly, pressure can alter the chemical characteristics of elements by activating the core electrons, the unoccupied orbitals and the quantum orbitals at the interstitial sites, leading to many new surprising phenomena. In this talk, I will outline the novel compounds and the new chemical phenomena within one conceptual framework based on the change of quantum states of electrons under high pressure. In contrast to the conventional view and chemical intuition, the quantum mechanics features of electrons such as directional bonds, inhomogeneous distribution, lower symmetry etc. are actually magnified by the increasing pressure, giving rise to rich moieties and variations in novel inorganic compounds. One striking example is that the core electrons can be activated to form bonds, violating a primary principle of chemistry. Other examples include electrons detaching from all atoms to play the role of anions at the interstitial sites (electrides), noble gases behaving as anions because their outer-shell d orbitals gain electrons, and noble gases reacting with ionic compounds without forming any chemical bonds. The influence of this new picture on future studies that is destined to higher pressures, more complex compositions and applicable materials is discussed.