Superionicity and Polymorphism in Calcium Fluoride at High Pressure

Abstract

We present a combined experimental and computational first-principles study of the superionic and structural properties of CaF2 at high P − T conditions. We observe an anomalous superionic behavior in the low-P fluorite phase that consists in a decrease of the normal → superionic critical temperature with compression. This unexpected effect can be explained in terms of a P -induced softening of a zone-boundary X phonon which involves exclusively fluorine displacements. Also we find that superionic conductivity is absent in the high-P cotunnite phase. Instead, superionicity develops in a new low-symmetry high-T phase that we identify as monoclinic (space group P 21/c). We discuss the possibility of observing these intriguing phenomena in related isomorphic materials.

Giant Mechanocaloric Effects in Fluorite-Structured Superionic Materials

“…At equilibrium (σ = 0), the adopted BMH potential provides a superionic critical temperature of 1350(50) K, which is in good accordance with reported experimental data and DFT calculations. 5 In view of such an agreement, we assume that the classical BMH and first-principles DFT results obtained in the rest of cases are also consistent. Figure 1b shows the stress dependence of T s calculated under broad σ > 0 (compressive) and σ < 0 (tensile) conditions.…”

Simulation and understanding of atomic and molecular quantum crystals

“…Next, we summarise the basic aspects of the most popular E xc [n] functionals found in computational studies of condensed matter systems and materials. Additional details on these topics can be found in recent and more specialized reviews (see, for instance, Perdew, 2013;Klimeš and Michaelides 2012;Dobson and Gould, 2012;Cazorla, 2015). We note that the current number of commercially available and open-source DFT computer packages is huge (at least in comparison to that of eQMC codes); a reference to some of them can be found, for instance, in Cazorla (2015).…”

“…This is the case, for instance, of crystals at extreme thermodynamic conditions (Loubeyre, 1987;Cazorla and Boronat, 2008a;Cazorla and Errandonea, 2014;Cazorla and Boronat, 2015b). A possible solution to overcome this modeling difficulty is to go beyond pairwise additivity, that is, to consider higher order terms in the approximation to the atomic interactions.…”

“…The cohesion between atoms in solid helium, for instance, is so weak that to a first approximation this crystal can be described by a system of hard spheres (Kalos, Levesque, and Verlet, 1974). Nevertheless, the description of longranged and very weak interactions poses a serious challenge to some families of first-principles methods (also known as ab initio because do not rely on any predetermined knowledge of the atomic forces) as the analytical expression of the corresponding electronic exchange and correlation energies are intricate and difficult to approximate for computational purposes (Klimeš and Michaelides, 2012;Cazorla, 2015). This circumstance converts quantum solids into an ideal playground in which to perform benchmark calculations for assessing the performance of standard and advanced electronic band-structure first-principles methods like, for instance, density functional theory (DFT) and electronic quantum Monte Carlo (eQMC) [Driver et al, 2010;Henning et al, 2010;Clay III et al, 2014;Clay III et al, 2016] (see Sec.…”

“…Metal hydrides are also promising for hydrogen storage applications (Grochala and Edwards, 2004;Shevlin and Guo, 2009) since they can supply large amounts of gas upon thermodynamic destabilisation. Likewise, carbon-based nanostructures (e. g., graphene, nanotubes, and fullerenes) exhibit large gas uptake capacities (Cazorla, Shevlin, and Guo, 2011;Gadipelli, 2015;Cazorla, 2015) as a consequence of their large surface-to-volume ratio, light atomic weight, and great thermodynamic stability. Diamonds with negatively charged nitrogen-vacancy centers, another type of carbonaceous nanomaterial, are playing a crucial role on the development of scalable quantum computing components (Fuchs et al, 2011;Nemoto et al, 2014).…”