Crystal structures and electronic properties of solid fluorine under high pressure**Project supported by the National Natural Science Foundation of China (Grant Nos. 51632002, 51572108, 11634004, and 11174102), Program for Changjiang Scholars and Innovative Research Team in University, China (Grant No. IRT 15R23), the National Fund for Fostering Talents of Basic Science, China (Grant No. J1103202), and the Development Program of

Abstract

As the previously proposed structures of C2/m and C2/c possess similar enthalpies and x-ray diffraction patterns, the space group of fluorine at ambient pressure is in controversy. We successfully obtain its thermodynamically stable low-pressure phase, which shares the same structure as the earlier known C2/c. Further investigations on phonon spectra reveal the instability of the C2/m structure with imaginary frequency in the Brillouin zone and confirm the dynamically stable property of the C2/c structure at the same time. Compressing fluorine up to 8 GPa, the C2/c phase is found to undergo a phase transition to a new structure with a space group of Cmca. Electronic energy band structures indicate the insulating feature of C2/c and Cmca with no bands across the Fermi level. The infrared (IR) and Raman spectra of C2/c and Cmca at selected pressures are calculated to provide useful information to future experiments.