Pressure-induced non-metallization of calcium in calcium bromide compounds

Calcium (Ca), a typical alkaline earth metal, generally exhibits a [Formula: see text]2 oxidation state under ambient pressure. In this study, we systematically investigated the structural evolution and electronic properties of the Ca–Br system in the pressure range of 0–1400[Formula: see text]GPa by combining first-principles calculations with the CALYPSO structure prediction method. Bader charge analysis reveals that increasing pressure drives Ca to transform from a conventional [Formula: see text]2-valence reductant into an oxidant with a negative oxidation state. Orbital occupancy calculations indicate that the Ca-3d orbitals are activated under high pressure, allowing them to capture electrons from the Br-4p orbitals and thereby exhibit anion-like electron-accepting behavior, leading to a reverse charge transfer phenomenon. Further projected density of states (PDOS) analysis shows that pressure-induced band broadening serves as the driving force for the non-metallization of Ca. This work not only uncovers the complex valence behavior of Ca under extreme conditions but also provides new theoretical insights into the non-metallization of main-group metals at high pressures.