# Comment on "High-pressure phases of group-II difluorides: Polymorphism and superionicity"

## TL;DR

This paper critiques a previous study on high-pressure phases of group-II difluorides, arguing that the quasi-harmonic approximation used is unreliable for superionic phase boundaries, supported by additional ab initio calculations.

## Contribution

It highlights limitations of the QHA method for studying superionic phases and provides additional ab initio calculations to support this critique.

## Key findings

- QHA-based phase diagram for CaF₂ is inconsistent with experimental data.
- QHA cannot reliably predict superionic phase boundaries.
- Additional calculations show limitations of QHA in superionic compounds.

## Abstract

Nelson et al. [Phys. Rev. B 95, 054118 (2017)] recently have reported first-principles calculations on the behaviour of group-II difluorides (BeF$_{2}$, MgF$_{2}$, and CaF$_{2}$) under high-pressure and low- and high-temperature conditions. The calculations were based on ab initio random structure searching and the quasi-harmonic approximation (QHA). Here, we point out that, despite the of inestimable value of such calculations at high-pressure and low-temperature conditions, the high-$P$ high-$T$ phase diagram proposed by Nelson et al. for CaF$_{2}$ neither is in qualitative agreement with the results of previous ab initio molecular dynamics simulations nor with the existing corps of experimental data. Therefore, we conclude that the QHA-based approach employed by Nelson et al. cannot be applied reliably to the study of phase boundaries involving superionic phases. This conclusion is further corroborated by additional ab initio calculations performed in the superionic compounds SrF$_{2}$, BaF$_{2}$, Li$_{3}$OCl, and AgI.