Pressure-induced second-order phase transition in fluorine

Area of Science:

• Solid-state chemistry
• Computational materials science
• High-pressure physics

Background:

• Understanding crystal structure stability is crucial for predicting material properties under extreme conditions.
• Fluorine's unique electronic properties, specifically the absence of sigma-holes, influence its intermolecular interactions compared to heavier halogens.
• The high-pressure behavior of molecular solids like fluorine remains an active area of research.

Purpose of the Study:

• To investigate the relative stability between the ambient alpha-F2 (C2/c) and a proposed high-pressure (Cmce) crystal structure of fluorine.
• To elucidate the mechanism and nature of the pressure-induced phase transition in fluorine.
• To compare the structural behavior of fluorine with heavier halogens under pressure.

Main Methods:

• Density Functional Theory (DFT) calculations using the PBE0+D3(ABC)/TVZP level of theory.
• Quantum Monte Carlo (QMC) calculations for further assessment of relative energies.
• Phonon dispersion spectra analysis to identify dynamical instabilities.

Main Results:

• The C2/c structure is energetically favored over the Cmce structure at ambient pressure.
• The Cmce phase exhibits a dynamical instability near the Gamma-point at ambient pressure, attributed to repulsive head-to-head interactions between fluorine molecules due to the absence of sigma-holes.
• This dynamical instability in the Cmce phase disappears with increasing pressure, facilitating the phase transition.

Conclusions:

• The pressure-induced phase transition from C2/c to Cmce in fluorine is confirmed to be of second-order.
• The absence of sigma-holes in fluorine is a key factor differentiating its high-pressure behavior from heavier halogens.
• Computational methods provide valuable insights into the complex phase behavior of elements under pressure.