Ultra-low concentration phase separation in solids: Ag in (Cd, Hg)Te

We show how phase separation, in the form of a redistribution of impurities (dopants in a semiconductor), can occur at impurity concentrations that ar e more than one order of magnitude lower than hitherto observed. This pheno menon results from the balance between long-range electrostatic repulsion a nd the elastic attraction of the dopants, which deforms the anisotropic hos t lattice. We observed such a phase separation for Ag in (Cd, Hg)Te at Ag c oncentrations < 0.02 at. %. This also leads to the formation of a thermodyn amically (as opposed to kinetically) stable p-n junction in the a-phase reg ion. Searching for phase separation at such low concentrations requires hig hly sensitive analyses, here made possible because of the difference in con ductivity type between the phases.