Ab initio molecular-dynamics simulations of superionic phases of Cu halides and Ag chalcogenides

The dynamic properties of mobile ions in the superionic conductors CuI and Ag2Sc are studied by ab initio molecular-dynamics simulations. The superionic behavior of these materials is successfully reproduced by our simulations. To investigate the bonding nature of these materials, the atomic charge of each ion and the bond order between mobile and immobile ions are calculated by the population analysis as a function of time. It is shown that, in CuI, the covalent bonding around the Cu ions weakens when they diffuse in the octahedron cage, and the ionicities for the Cu ions at the octahedral sites are larger than those for the Cu ions at the tetrahedral sites. For Ag2Se, we demonstrate that the bond order for the mobile Ag ions becomes smaller and their atomic charges become larger accompanied with the diffusive motion. These results reveal that the high ionic conductivity is the outcome of the fluctuation of bonding properties in the materials, and agree well with the predictions of the bond fluctuation model proposed for the conduction mechanism of the mobile ions in the superionic conductors.