Structure and dynamics of superionic conductors. II. Collective excitations and single-particle motion

Molecular-dynamics calculations have been performed for a two-dimensional disordered many-particle system with strong anharmonicities which is embedded in a periodic potential. The disordered system is formed by 128 silver ions and the periodic potential is given by a well-defined iodine lattice also consisting of 128 ions. Although restricted to a two-dimensional model, the results for the main dynamical properties of the system agree well with various experimental data for $\alpha$-AgI. Scattering functions have been used in order to study the particle motion. It turned out that the shape of the coherent scattering law can be interpreted in terms of jump-diffusion processes. Furthermore, we have found that the individual motion is strongly coupled to the collective modes (in particular optical modes) of the system.