Crystal Structure Evolution of Fluorine under High Pressure

Abstract

Fluorinated compounds in the last decade were applied as photo-thermo-refractive glasses, high-stress lubricants, and pharmaceutical drugs due to their good mechanical properties and biocompatibility. Although fluorinated materials are largely employed, the possibility of predicting new structures was limited by the impossibility to use density functional theory (DFT) to describe interatomic and intermolecular interactions correctly. This is seen linearly to increase with fluorine concentration. In crystal structure prediction, modern algorithms are usually combined with first-principles methods employed for global solution, which sometimes fail to describe systems as in the case of strongly correlated materials. Fluorine is one of the tricky elements, which is characterized by relativistic effects and no overlap between the DFT exchange hole and the exact exchange hole. Thus, no relativistic exchange-correlation functional was seen to adequately describe fluorine. In this work, we have found an excellent compromise to investigate fluorinated materials using a combination of SCAN (exchange) and rVV10 (correlation) functionals. This was found through the fundamental study of α- and β-fluorine phases, showing α-fluorine as the most stable structure at temperatures lower than 35 K and 0 GPa with respect to β-fluorine. Further, we have computed crystal structure evolution under pressure looking for new stable fluorine allotropes using the USPEX evolutionary algorithm coupled with the SCAN-rVV10 exchange-correlation functional discovering two phase transitions: one from C2/c (i.e., α-fluorine) to Cmca at ∼5.5 GPa and from Cmca to the P4¯ 21c phase at 220 GPa; all these structures possess metallic behavior. The achievements of this work lie far beyond the thermodynamic of fluorine crystals under pressure. It will give the right instrument to understand the chemical behavior of fluorinated materials under pressure with consequent great speed up to the crystal structure prediction of fluorinated and fluorine-based materials.