Pressure-induced second-order phase transition in fluorine.

The relative stability between the crystal structure of α-F 2 , space group C 2/ c , and a hypothesized high-pressure phase, space group Cmce , was explored using Density Functional Theory at the PBE0+D3(ABC)/TVZP level of theory and further assessed by Quantum Monte Carlo calculations. The analysis of the phonon dispersion spectra reveals that, at ambient pressure, besides the energy difference favoring the C 2/ c structure, the Cmce phase also presents a dynamical instability near the Γ -point, which disappears with increasing pressure. The unstable vibrational mode can be related to the absence of σ-holes in the fluorine molecule, which renders a repulsive head-to-head interaction between molecules, as opposed to heavier halogens, in which the presence of σ-holes stabilizes the orthogonal Cmce structure. The results show that the pressure-induced phase transition C 2/ c → Cmce is of second-order.