Ground state dynamically stable phases for fluorine in the TPa pressure regime by evolutionary algorithms

TL;DR

This study uses ab initio methods and evolutionary algorithms to identify stable high-pressure phases of fluorine in the TPa range, revealing a direct transition from molecular to extended structures.

Contribution

It provides new insights into fluorine's high-pressure phases, confirming a direct molecular to extended structure transition and challenging previous reports of intermediate phases.

Findings

Transition from molecular $Cmca$ to $Pmar{3}n$ structure at 2.7 TPa

Disagreement with previous studies on intermediate phases

Identification of stable extended structures at high pressure

Abstract

In this work, we employed \textit{ab initio} methods combined with evolutionary algorithms for searching stable structures for fluorine in the terapascal (TPa) regime. We performed several structural searches using the USPEX code, at pressures that spanned from 1 to 5 TPa and considered up to 16 atoms per cell for selected pressures. Our findings partially support recent studies by validating the transformation of fluorine from a molecular form, $Cmca$, into an intermediate polymeric form before its eventual dissociation. In fact, the enthalpy comparisons between candidate structures of fluorine at high pressure show a direct transition from the molecular phase $Cmca$ into a $Pm\bar{3}n$ extended structure at 2.7 TPa, the later consisting of linear chains and independent atoms, which disagrees with previous conflicting reports that proposed other intermediate phases to also exist as stable crystalline forms close to 3 TPa.