Electronegativity and chemical hardness of elements under pressure

Abundant evidence has shown the emergence of exotic chemical phenomena under pressure, including the formation of unexpected compounds and strange crystal structures. In many cases, there is no convincing explanation for these phenomena, and there are virtually no chemical rules or models capable of predicting or even rationalizing these phenomena. Here, we calculate, as a function of pressure, two central chemical properties of atoms, electronegativity and chemical hardness, which can be seen as the first- and second-order chemical potentials. Mulliken electronegativity, which is the negative of the chemical potential of the electron in a given atom relative to the vacuum, is appropriately modified; instead of taking the vacuum (impossible under high pressure), we take the homogeneous electron gas as reference. We find that for most elements, chemical hardness and electronegativity decrease with pressure, consistent with pressure-induced metallization. Furthermore, we discover that pressure-induced s-d orbital transfer makes Ni, Pd, and Pt “pseudo–noble-gas” atoms with a closed d-shell configuration, and the elements preceding them (Fe and, especially, Co, Rh, and Ir) electron acceptors, while the elements right after them (Cu, Ag, Zn, and Cd) become highly electropositive. We show the explicative and predictive power of our electronegativity and chemical hardness scales.