Ionic diffusion within the α* and β phases of Ag3SI

The ionic diffusion mechanism of mobile ions within an underlying bodycentred cubic ( bcc) sublattice of immobile counterions is discussed. In particular, the case of equal numbers of two ionic species forming long- range ordered and disordered bcc arrays is considered, since these form the basis of the cubic perovskite and alpha- AgI- type crystal structures, respectively. Their structural behaviour, and its influence on the dynamic ionic disorder which characterizes superionic conduction, is illustrated for the case of Ag+ diffusion within the beta and alpha* phases of Ag3SI. The calculated behaviour obtained by molecular dynamics ( MD) computer simulations is validated with reference to published neutron diffraction and ionic conductivity measurements of Ag3SI, and used to examine the preferred diffusion pathways. The relevance of these findings for the anion conduction mechanisms within perovskite structured compounds is briefly discussed.