STRUCTURAL TRANSFORMATION AND SUPERIONIC BEHAVIOR OF A HIGH ANGLE GRAIN BOUNDARY IN CaF2 : A COMPUTER SIMULATION STUDY

Overview & Synopsis

We present the results of a molecular dynamics study of diffusion in a Σ=5 [001] (210) tilt grain boundary in CaF2 using a rigid-ion potential. We found by minimization that the coincidence position corresponds to the lowest energy structure of the boundary. This structure remains stable up to T≈0.2 Tmeiting, a temperature above which a local structural transition occurs. For temperatures T⩾0.44 Tmelting, enhanced anionic diffusion is observed possibly related to this transformation. The critical temperature for superionic conduction in the grain boundary, TGBc⩽750 K, is substantially lower than in the bulk, Tc=1400 K. The analysis of the atomic trajectories reveals the existence of a variety of cooperative diffusion mechanisms originating this premature intergranular superionic behavior.