High-pressure crystal structure and properties of BrCl

The pressure-induced phase transitions of bromine chloride (BrCl) were studied using the CALYPSO structure prediction method and first-principles calculations. Geometry optimization showed that the BrCl underwent the following phase transitions under high pressure: Cmcm → C2/m → Fddd → P4/mmm. These phase transitions occurred at 75, 94, and 140 GPa, respectively. All four phases were calculated to be dynamically stable, judging by the absence of any imaginary phonon modes in the whole Brillouin zone. As the pressure increased, the number of bonds between Br atom and surrounding Cl atoms increased. The high-pressure P4/mmm-BrCl phase (stable at 150 GPa) consisted of eight-coordinated Br atoms and is predicted to exist in BrCl compounds for the first time. The band overlap of the Cmcm phase, which caused the pressure-induced semiconductor → metal transition, occurs ∼50 GPa. This article lays the foundation for further theoretical and experimental research.