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

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.

References in corpus (4)

• Ab initio melting curve of molybdenum by the phase coexistence method
• Superionicity and Polymorphism in Calcium Fluoride at High Pressure
• Constraints on the phase diagram of molybdenum from first-principles free-energy calculations
• High-pressure phases of group II difluorides: polymorphism and superionicity

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