COLLECTIVE SURFACE-DIFFUSION ON TRIANGULAR AND SQUARE INTERACTING LATTICE GASES

The collective (or chemical) diffusion in interacting repulsive lattic e gases is studied both in triangular and square symmetries. The effec t of lateral interactions at the saddle point position is taken into a ccount. The collective diffusion coefficient D-c is calculated within the generalized Darken equation (GDE). Under this approximation memory effects are neglected and D-c is expressed as the ratio of the averag e jump rate [w] and of the zero-wave-vector static structure factor S( 0). Both quantities are calculated by the cluster variation method wit h clusters of seven (in the triangular lattice) and nine (in the squar e lattice) sites. It is shown that D-c has a very complicated behaviou r in the regions of the phase diagrams where ordered phases are formed . This happens at coverages around theta = 1/3 and 2/3 in the triangul ar lattice and around theta = 1/2 in the square lattice. The lateral i nteractions at the saddle point have strong quantitative effects on D- c, but many qualitative features are preserved. In the square lattice case, our results are compared to those of Monte Carlo simulations by Uebing and Gomer, with excellent agreement. This shows that memory eff ects are not of great importance even at low temperatures. (C) 1998 El sevier Science B.V. All rights reserved.