High-pressure CaF2 revisited: A new high-temperature phase and the role of phonons in the search for superionic conductivity

Abstract

We recently proposed a high-pressure and high-temperature P 62m-symmetry polymorph for CaF2 on the basis of ab initio random structure searching and density-functional theory calculations [Phys. Rev. B 95, 054118 (2017)]. We revisit this polymorph using both ab initio and classical molecular dynamics simulations. The structure undergoes a phase transition to a superionic phase in which calcium ions lie on a bcc-symmetry lattice (space group Im3m), a phase not previously discussed for the group-II difluorides. We demonstrate that modelling this phase transition is surprisingly difficult, and requires very large simulation cells (at least 864 atoms) in order to observe correct qualitative and quantitative behaviour. The prediction of superionic behaviour in P 62m-CaF2 was originally made through the observation of a lattice instability at the harmonic level in DFT calculations. Using superionic α-CaF2, CeO2, β-PbF2 and Li2O as examples, we examine the potential of using phonons as a means to search for superionic materials, and propose that this offers an affordable way to do so. II. METHODSAIMD simulations in this paper use the cp2k code [32] and density-functional theory (DFT) with the PBE exchange-correlation functional [33]. Goedecker-Teter-arXiv:1811.08740v1 [cond-mat.mtrl-sci]

Superioniclike Diffusion in an Elemental Crystal: bcc Titanium

“…We can make yet another analogy between the behavior of bcc Ti and type-II superionics. A particular phonon mode of B1u symmetry appears to be connected to the superionic behavior in type-II superionic transitions [54][55][56][57][58]. The atomic displacements in this mode are closely connected to diffusion processes in fluorite crystal structures, analogous to the case of w-mode displacements/concerted-migration in bcc Ti.…”

High-Pressure Polymorphs of LaHO with Anion Coordination Reversal

“…This phase will be denoted as α-LaHO by analogy with predicted CaF 2 polymorphs. 23 α-LaHO contains three independent lanthanide sites, all in 8-fold (H × 4, O × 4) and nearly cubic environments. 22 As reported previously, from the anion-centered viewpoint, both anions lie in tetrahedral environments, but with distinct volumes of 8.15 Å 3 for HLa 4 and 7.39 Å 3 for OLa 4 .…”

“…We call the 3 GPa phase as γ-LaHO by analogy with CaF 2 polymorphs. 23 As for multiple anion system, Glaẗzle et al recently reported that LnOF oxyfluorides (Ln = Nd, Sm, Eu) adopt the PbCl 2 -type structure when synthesized under pressure (11 GPa). 27 The observed pressure-induced transition (α→γ) in LaHO is understandable from various angles.…”

“…The cell parameters of a = 8.0765(2) Å and c = 5.7257(2) Å are quite close to those of reported. This phase will be denoted as α-LaHO by analogy with predicted CaF 2 polymorphs . α-LaHO contains three independent lanthanide sites, all in 8-fold (H × 4, O × 4) and nearly cubic environments .…”

Calcium fluoride as high-k dielectric for 2D electronics

“…In this regard, theoretical methods, for example DFT calculations, should be employed to clarify the type of interface between CaF 2 and different 2D materials. So far, the structure of CaF 2 has been investigated by numerous simulation methods at high pressure, 121 high temperature, 122 and different phases. 123,124 The formation of local defects in CaF 2 has been studied using periodic density functional theory (PDFT) 125 and periodic electrostatic embedded cluster method (PEECM), 126 revealing that the formation of F vacancies is favorable for small electron chemical potentials in the CaF 2 bulk.…”