DIFFUSION IN DISORDERED STRUCTURES

A lattice gas model is developed to study the influence of site and sa ddle point enthalpy disorder on tracer and collective diffusion of par ticles. This model can be applied to interstitial diffusion as well as substitutional diffusion processes. The short-range and long-range tr acer dif fusion coefficients are evaluated from Monte Carlo Simulation s (MCS) for particle concentrations 0 less than or equal to c < 1 and compared with Effective Medium Calculations (EMA). The results for sho rt-range diffusion agree rather well to theoretical calculations for a ny particle concentration and enthalpy disorder. The long-range diffus ion coefficient determined by MCS corresponds to results obtained by E MA for low particle and high particle concentrations. The influence of energetical disorder on the tracer correlation factor is investigated for different particle concentrations. In case of low particle concen trations no dependence of the tracer correlation factor on the width o f the site enthalpy distribution is found. In contrast, the tracer cor relation factor decreases by increasing the width of the distribution of the saddle point enthalpies caused by increasing static correlation s. With increasing particle concentrations the tracer correlation fact or decreases. A complex interaction between static and dynamic correla tions is found in the case of high particle concentrations c --> 1. Th e tracer and the collective diffusion coefficient could be determined as a function of particle concentration as well as the width of the di stribution of the site and saddle point enthalpies.