Prediction of Core Electron Reactivity and High Oxidation States in Radium under Pressure

Recent first-principles calculations and crystal structure predictions revealed a striking phenomenon that the 5p core electrons of Cs can be activated and can form covalent bonds with F under high pressure, causing the formation of unconventional CsFn (n > 1) compounds, in which Cs can be oxidized up to +5 state. Despite many efforts, CsFn remains the only example where the core electrons of the main group element become reactive. Here, we conduct an extensive crystal structure and stability prediction on Ra-F compounds under high pressure, which successfully identifies a series of stable compounds with high F stoichiometry (RaFn, n = 3-8). By various electronic structure and bonding feature analyses, we demonstrate that the 6p core electrons of Ra are activated in most of these compounds and are involved in forming covalent bonds with neighboring F atoms. Many novel phenomena emerge in RaFn compounds when the core electrons are activated, including oxidation states as high as +8, covalent Ra-F bonds, high-pressure molecular crystals, mixed valency in RaF3 and RaF4 compounds, and high coordination numbers, all of which point to the striking fact that Ra becomes a very different element under high pressure and exhibits very rich chemistry.

Automated summary

Recent research has shown that the core electrons of Cs and Ra can be activated under high pressure, leading to the formation of unconventional compounds with F. This activation results in significant changes in the properties of Cs and Ra, exhibiting rich chemistry.