Electron-Rich Multicenter Bonding in Elemental Crystals: A High-Pressure Study

An unconventional bond has been recently proposed to be present in the crystalline phases of phase change materials (PCMs), that has been quoted as electron-rich multicenter (ERM), hypervalent, 3 center-4 electron, resonant, or metavalent bond. It has also been suggested that this bond occurs at high pressure in IV-VI and V2-VI3 compounds that are not PCMs at room pressure, and that are characterized by the combination of a primary covalent ppσ-bond and a secondary bond in which the lone electron pairs (LEPs) are involved. Since the same bonding scenario occurs in most pnictogens and chalcogens at room pressure, our theoretical calculations show that group-V (As, Sb, Bi) and -VI (Se, Te) elements develop ERM bonds under compression as the sixfold coordination is approached, being polonium the only element whose crystalline α and β structures exhibit ERM bonds at room pressure. The ERM bond formation mechanism and how it depends on the type of LEP present in secondary bonds is reported. Since comprehensive understanding of chemical bonding in materials plays a crucial role in explaining structure and physico-chemical properties as well as to envisage applications, this work paves the way for a better knowledge and use of advanced materials.