MOLECULAR-DYNAMICS SIMULATION OF IONIC-CONDUCTIVITY IN THE FLUORIDE-PEROVSKITE KCAF3

Constant-temperature constant-volume molecular dynamics simulations we re performed for the fluoride-perovskite KCaF3. Ionic conductivity val ues were predicted using systems designed so as to simulate crystals c ontaining extrinsic defects; these values were compared with experimen tal measurements made between 575 and 690 K. At these temperatures, ex trinsic defects were found to be prerequisite for diffusion of fluorin e ions. During defect-free simulations, intrinsic disorder was not gen erated spontaneously. Fluorine migration precedes via a discrete vacan cy ''hopping'' mechanism, in which the anion migration time is of at l east an order of magnitude less than the average residence time at a g iven lattice site. This is in contrast to the partially correlated mec hanism observed at temperatures approaching the simulated melting poin t for KCaF3, in which vacancies are generated by the formation of fluo rine ion pairs occupying interstitial sites.