Conductivity of a Three-Component "Reactive" Percolation Model

We consider a new kind of percolation model in which sites can be occupied by two kinds of "atoms," $A$ or $B$, or by "molecules" formed from $A$ and $B$. Introducing plausible models for conductivity we find numerically that the total sample resistivity $\rho$ can be a nonmonotonic function of the total concentration of free and bound $A$. These results are qualitatively consistent with recent experiments on solid mixtures of sodium and ammonia. We discuss other features of the model qualitatively.