KINETICS OF ORDERING AND DISORDERING FOR A 2-DIMENSIONAL BINARY LATTICE-GAS

We study the coverage and stoichiometry dependence of the kinetics of ordering and disordering for a two-dimensional binary lattice gas with repulsive interactions using a time-dependent quasichemical approxima tion. The system is characterized by three long-range order (LRO) para meters and three short-range order (SRO) parameters. The time evolutio ns of all six quantities are obtained by numerical solution of appropr iate coupled non-linear equations of motion. For the case of ordering, the SRO typically reaches a quasi-equilibrium state very rapidly, fol lowed by exponential growth of LRO and final relaxation of both SRO an d LRO to equilibrium. For the case of disordering, the SRO typically a chieves its equilibrium value quickly while the LRO decreases very rap idly at short times followed by an exponential decay to the final stat e. In all cases, the exponential time constant is obtained analyticall y from a linearized version of the theory and qualitative arguments ar e advanced to explain its variations as a function of coverage and sto ichiometry at T = 1/2T(C).