We show that percolation can control not only diffusion in solids, but in the case of semiconductors also their electrical activity, via the doping action of the diffusing species. This occurs in (Hg1-xCdx)Te (MCT) when xCd < 0.8. The 107 times higher diffusivity at xCd < 0.8 can be understood by realizing that the percolation threshold for an ideal FCC lattice is at 0.19. While normally Ag is a donor, it can be an acceptor by stabilizing the Hg(I) state. This is possible by interaction with 2 Hg neighbors, a process that will be favorable above the Hg percolation limit. The fast Ag diffusion also holds the clue for the occurrence of ultra-low concentration phase separation in this system, the result of a balance between elastic attraction and Coulombic repulsion between the charged dopants. Prima facie evidence for this phase separation comes from coulometric Ag titration in and out of MCT.
Bibliographical noteFunding Information:
We thank the Israel Science Foundation for partial support, T. Skauli (Norwegian Defence Research Establishment) for making his results with Hg-rich films and his literature compilation available to us (Ref.  ), R. Granek (Weizmann Inst.) and I. Riess (Technion) for stimulating discussions.
- Phase transition