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 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.