High-Pressure, High-Temperature Phase Diagram of Calcium Fluoride from Classical Atomistic Simulations

Abstract

We study the phase diagram of calcium fluoride (CaF2) under pressure using classical molecular dynamic simulations performed with a simple pairwise interatomic potential of the Born-Mayer-Huggings form. Our results obtained under conditions 0 ≤ P 20 GPa and 0 ≤ T 4000 K reveal a rich variety of multi-phase boundaries involving different crystal, superionic and liquid phases, for all which we provide an accurate parametrization. Interestingly, we predict the existence of three special triple points (i.e. solid-solid-superionic, solid-superionic-superionic and superionicsuperionic-liquid coexisting states) within a narrow and experimentally accessible thermodynamic range of 6 ≤ P ≤ 8 GPa and 1500 ≤ T ≤ 2750 K. Also, we examine the role of short-ranged repulsive (SR) and long-ranged attractive (LA) atomic interactions in the prediction of melting lines with the finding that SR Ca-F and LA F-F contributions are most decisive.

High accuracy and transferability of a neural network potential through charge equilibration for calcium fluoride

“…Furthermore, to provide a comparison between the results of the CENT potential and a FF, the BMH form of Buckingham potential was used as implemented in the GULP code 47 and parametrized according to Ref. [48]. Note that the BMH potential was originally fitted to bulk properties and is thus expected to perform well for periodic systems.…”

Superionicity and Polymorphism in Calcium Fluoride at High Pressure

“…This remarkable agreement between theory and experiments certifies that the reported P -induced T s anomaly constitutes a genuine effect. We note that none of the classical simulation works performed to date have reported any such peculiar behavior [19,20], thus we may conclude that the employed interaction models are unsuitable to emulate CaF 2 at high P − T conditions. In the search to rationalize the origins of the observed T s anomaly, we turned our attention onto collective phonon excitations [13][14][15].…”

“…The detailed ionic processes sustaining superionicity in this high-P γ-phase, however, are totally uncertain [18]. As for theory, only few simulation works have recently addressed the description of ionic conductivity at nonambient conditions [19,20]. Those few computational studies however rely all on molecular dynamics simulations performed with semi-empirical pairwise potentials which have been tuned to reproduce the behavior of CaF 2 at ambient pressure.…”

“…In our AIMD simulations, we identified ionic conductivity by inspecting the calculated mean squared displacement ∆r 2 (t) of the Ca +2 and F − ions [20]. To ascertain that the crystal remained vibrationally stable (i.e., the thermal average position of each ion remains centered on its perfect-lattice site), we computed the position cor-…”

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

“…[18]), however, these data have not yet structurally characterized this phase. Theoretical work has proposed that γ-CaF 2 melts directly at high temperature [19], becomes superionic at high temperature (in the same structure) [20], or * jn336@cam.ac.uk undergoes a phase transition to another solid phase which then becomes superionic [17]. Our recent study proposed a P 62m-symmetry CaF 2 structure as a high-T modification of γ-CaF 2 [5] (Fig.…”