Predicting Halogen Crystal Structures at Megabar Pressures

There is a huge variety of minerals in the earth and many of their properties at high pressures have not yet been characterized. Fluorine and chlorine are both highly reactive and volatile elements. For this reason, using diamond anvil cells and x-ray diffraction to examine their crystal structure under high pressure has been challenging. The difficulty of determining their high pressure phase with laboratory experiments makes it an attractive problem for computational methods like density functional theory (DFT). In order to efficiently generate viable candidate structures we extended our symmetry-driven structure search (SYDSS) algorithm introduced by R. Domingos et al. [Phys. Rev. B 98, 174107 (2018)] to generate molecular configurations. We added a simplex optimization routine to the algorithm based on the Nedler-Mead function minimization method in order to generate possible structures which were not only symmetric, but which also respected the known diatomic character of chlorine and fluorine molecules. Armed with these new methods we were able to generate thousands of symmetric and molecular crystal structures containing between 2 and 16 atoms per cell that were sampled from all 230 space groups. Using DFT calculations to relax these randomly generated structures across pressures ranging from 0.1 to 10 Mbar. we were able to efficiently predict novel crystal structures for both fluorine and chlorine within this pressure range. We discuss their properties in this presentation.