  Transport phenomena are studied for a binary (AB) alloy on a rigid square
lattice with nearest-neighbor attraction between unlike particles, assuming a
small concentration $c_v$ of vacancies $V$ being present, to which $A(B)$
particles can jump with rates $\Gamma_A (\Gamma_B)$ in the case where the
nearest neighbor attractive energy $\epsilon_{AB}$ is negligible in comparison
with the thermal energy $k_BT$ in the system. This model exhibits a continuous
order-disorder transition for concentrations $c_A,c_B=1-c_A-c_V$ in the range
$c_{A,1}^{crit}\leq c_A \leq c_{A,2}^{crit}$, with
$c_{A,1}^{crit}=(1-m^*-c_V)/2$, $c_{A,2}^{crit} =(1+m^*-c_V)/2$, $m* \approx
0.25$, the maximum critical temperature occurring for $c*=c_A=c_B=(1-c_V)/2$,
i.e. $m^*=0$. This phase transition belongs to the $d=2$ Ising universality
class, demonstrated by a finite size scaling analysis. From a study of
mean-square displacements of tagged particles, self-diffusion coefficients are
deduced, while applying chemical potential gradients allow the estimation of
Onsager coefficients. Analyzing finally the decay with time of sinusoidal
concentration variations that were prepared as initial condition, also the
interdiffusion coefficient is obtained as function of concentration and
temperature. As in the random alloy case (i.e., a noninteracting ABV-model) no
simple relation between self-diffusion and interdiffusion is found. Unlike this
model mean field theory cannot describe interdiffusion, however, even if the
necessary Onsager coefficients are estimated via simulation.

Statistical Mechanics;Materials Science