Can percolation control doping, diffusion and phase segregation in (Hg,Cd)Te?

We show that percolation can control not only diffusion in solids, but in t he case of semiconductors also their electrical activity, via the doping ac tion of the diffusing species. This occurs in (Hg1-xCdx)Te (MCT) when x(Cd) < 0.8 The 10(7) times higher diffusivity at x(Cd) < 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 th e Hg(I) state. This is possible by interaction with 2 Hg neighbors, a proce ss that will be favorable above the Hg percolation limit. The fast Ag diffu sion also holds the clue for the occurrence of ultra-low concentration phas e separation in this system, the result of a balance between elastic attrac tion and Coulombic repulsion between the charged dopants. Primafacie eviden ce for this phase separation comes from coulometric Ag titration in and out of MCT. (C) 1999 Elsevier Science B.V. All rights reserved.