Diffusion in Disordered Structures

Abstract

A lattice gas model is developed to study the influence of site and saddle point enthalpy disorder on tracer and collective diffusion of particles. This model can be applied to interstitial diffusion as well as substitutional diffusion processes. The short-range and long-range tracer diffusion coefficients are evaluated from Monte Carlo Simulations (MCS) for particle concentrations 0 ≤ c < 1 and compared with Effective Medium Calculations (EMA). The results for short-range diffusion agree rather well to theoretical calculations for any particle concentration and enthalpy disorder. The long-range diffusion coefficient determined by MCS corresponds to results obtained by EMA 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 concentrations no dependence of the tracer correlation factor on the width of the site enthalpy distribution is found. In contrast, the tracer correlation factor decreases by increasing the width of the distribution of the saddle point enthalpies caused by increasing static correlations. With increasing particle concentrations the tracer correlation factor decreases. A complex interaction between static and dynamic correlations is found in the case of high particle concentrations c → 1. The tracer and the collective diffusion coefficient could be determined as a function of particle concentration as well as the width of the distribution of the site and saddle point enthalpies.