Molecular Dynamics Simulation of CaF₂ Structure in Crystalline, Superionic and Molten Phases

Abstract

We describe here a number of molecular dynamics simulations on calcium fluoride over a range of temperatures spanning the transitions to the superionic and molten state. The simulation temperatures are 1400, 1590, 1800, 2000, 2200 K. By using the bond spherical harmonics method with equal neighbor number, we have studied the structure and bond orientation of cation sublattice and anion sublattice in superionic conductor CaF₂. The bond order parameters Q₁ have been calculated both for the computer generated instataneous configurations from the simulation system and for the standard configurations from the normal distribution model of bond orientation. The comparison of Q₁ between the molecular dynamics simulation and the normal distribution model shows that not only the cation sublattice but also the anion sublattice can be described by the normal distribution model. The cations keep their original fcc frame, but in the anion case there is a great deal of random distortions from the original anion sublattice.

Key Words:

• molecular dynamics simulation
• calcium fluoride
• superionic phase
• molten salt
• bond order parameters