COMPUTER-SIMULATION OF HYDROGEN DIFFUSION AND NUCLEAR MAGNETIC-RELAXATION ON A DISORDERED LATTICE

The dipolar nuclear magnetic relaxation rate associated with the hoppi ng diffusion of interstitial hydrogen atoms in a disordered alloy is c alculated by Monte Carlo methods. The principal features of the model system are that the atoms hop on a spatially disordered array of traps and the trapping energy varies from trap to trap so that the diffusio n of the hydrogen is characterized by a distribution of jump rates. Th e effective jump rate from a trap is assumed to have an Arrhenius depe ndence on temperature causing the distribution of jump rates to depend on temperature. Unlike earlier work, the method fully explores the wa y in which this dependence affects the mean jump rate as well as provi ding the means to calculate the relaxation as a function of both Larmo r frequency and temperature. The mean jump rate is found to deviate fr om the Arrhenius form in a manner that depends on the concentration of the hydrogen nuclear spins. At a given temperature the characteristic peak in the relaxation rate, which occurs in ordered solids when the product of the average jump rate and the Larmor frequency is approxima tely unity, is broadened, becomes asymmetric and is shifted in frequen cy by the presence of the jump rate distribution. The broadening is fo und to be less apparent when the relaxation rate is calculated as a fu nction of temperature but the asymmetry remains.