Pressure-induced second-order phase transition in fluorine

The relative stability between the crystal structure of α-F₂, space group C2/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 C2/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 C2/c → Cmce is of second-order.