Migration and correlation in highly defective systems: Fast-diffusion in lithium oxide

The high-temperature superionic phase of lithium oxide is characterized by a high concentration of Frenkel defects and a diffusion mechanism involving several types of atomic jumps. We have calculated the tracer-correlation factor and analyzed the migration paths of the Li ions obtained by molecular dynamics (MD). A kinetic Monte Carlo code, simulating the lithium vacancy diffusion, has been developed and used to predict the correlation factor as a function of the atomic fraction of defects. There is a good agreement with the result directly obtained by MD. The analysis of the jump paths shows that the direct exchange between a vacancy and a migrating atom is the main part of the diffusion mechanism. The other atomic jumps, although complex, mostly imply vacancies. The Li+ fast-diffusion proceeds by a vacancy mechanism involving several jump types.