MOLECULAR-DYNAMICS SIMULATION OF CAF2 STRUCTURE IN CRYSTALLINE, SUPERIONIC AND MOLTEN PHASES

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, 15 90, 1800, 2000, 2200 K. By using the bond spherical harmonics method w ith equal neighbor number, we have studied the structure and bond orie ntation of cation sublattice and anion sublattice in superionic conduc tor CaF2. The bond order parameters Q(l) have been calculated both for the computer generated instantaneous configurations from the simulati on system and for the standard configurations from the normal distribu tion model of bond orientation. The comparison of Q(l) between the mol ecular dynamics simulation and the normal distribution model shows tha t not only the cation sublattice but also the anion sublattice can be described by the normal distribution model. The cations keep their ori ginal fcc frame, but in the anion case there is a great deal of random distortions from the original anion sublattice.