Chemistry under high 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-dimentional 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 to future studies that is destined to
higher pressures, more complex compositions and applicable materials is discussed.
Reference
[1] M. S. Miao*, Y. Sun, E. Zurek, H. –Q. Lin, “Chemistry under high pressure”, Nature
Reviews Chemistry 4, 508 – 527 (2020).