Insights into the high-pressure behavior of solid bromine from hybrid density functional theory calculations

Abstract

Understanding the properties of molecular solids at high pressure is a key element in the development of new solid-state theories. However, the most commonly used generalized gradient approximation of the density functional theory (DFT) often fails to describe the behavior of these systems correctly at high pressure. Here we utilize the hybrid DFT approach to model the properties of elemental bromine at high pressure. The calculations reproduce in very good agreement with experimentation the properties of molecular phase I ($Cmca$ symmetry) and its pressure-induced transition into nonmolecular phase II $(Immm)$. The experimentally yet unobserved transition into phase III $(I4/mmm)$ is predicted to occur at 128 GPa, followed by subsequent formation of an fcc lattice at 188 GPa. Analysis of the structure and electronic properties of the modeled phases indicates that the molecular $Cmca$ phase becomes metallic just at the borderline of its stability, and that both the $Immm$ and $I4/mmm$ phases are metallic and quasi--two-dimensional. Finally, we show that the incommensurate phases of bromine postulated from experimentation are transient species that can be viewed as intermediates in the dissociation process occurring at the boundary of the transition from phase I to phase II.