Reply to “Comment on ‘High-pressure phases of group-II difluorides: Polymorphism and superionicity’ ”

Abstract

Cazorla et al. [preceding comment] criticize our recent results on the high-P T phase diagram of CaF2 [Phys. Rev. B 95, 054118 (2017)]. According to our analysis, Cazorla et al. have not converged their calculations with respect to simulation cell size, undermining the comment's conclusions about both the high-T behaviour of the P 62m-CaF2 polymorph, and the use of the QHA in our work. As such, we take this opportunity to emphasise the importance of correctly converging molecular dynamics simulations to avoid finite-size errors. We compare our quasiharmonic phase diagram for CaF2 with currently available experimental data, and find it to be entirely consistent and in qualitative agreement with such data. Our prediction of a superionic phase transition in P 62m-CaF2 (made on the basis of the QHA) is shown to be accurate, and we argue that simple descriptors, such as phonon frequencies, can offer valuable insight and predictive power concerning superionic behaviour.

Phonon–Ion Interactions: Designing Ion Mobility Based on Lattice Dynamics

“…[89] Here, the modes responsible for the jump soften and disappear across the superionic phase transition. [89] Similar phenomena have also been observed computationally in various oxygen-ion [117] and fluoride-ion [118] conductors having fluorite structure where a softening of a specific phonon mode is associated with the type II superionic transition in these materials. Other suggestions of phononic influences include specifically the softening of phonon modes at superionic phase transitions; [119,120] however, experimental observation of mode softening is experimentally challenging as it involves single crystals and momentum-resolved phonon dispersion curves.…”

“…[ 149 ] 3)While not in‐depth discussed here, the paddle‐wheel mechanism had been invoked to affect ionic motion decades ago and has recently been explored in newer materials systems. [ 83–123 ] Although it seems now clear that there are polyhedral rotations at least in amorphous solids that interact with the moving ions, a clear benefit and the understanding on how it really affects the ionic conductivity is still missing. Lattice vibrations and rotations are thermally excited processes and hence it seems likely that a random thermal motion can help a jumping ion if it occurs concurrently along a field direction.…”

Surface reconstructions and premelting of the (100) CaF2surface

“…Such behavior of CaF 2 in bulk phase has been reported and studies by some groups. [45][46][47][48] Based on having such knowledge, the MD simulations of bulk phase were done according to the setting as was described in Sec. II B.…”

Lithium diffusion in Li2X ( X=O , S, and Se): Ab initio simulations and inelastic neutron scattering measurements

“…The use of solid electrolyte between the electrodes has enabled to solve the issue of dendrite formation and leakage of electrolyte [1,3]. An extensive amount of research is underway to predict and design solid ionic conductors with ionic conductivity in the range of that of liquids [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. They are called the superionic conductors and possess an extremely low potential energy barrier for diffusion in the solid-state.…”