Metastable Percolation Phase Formation in Lif Implanted with Alkali Ions

Abstract

The formation of a continuous metallic layer in the doped region of LiF crystals implanted at low temperature, has been explained by the formation of bridges between next neighbouring alkali ions of the lattice around-each implanted ion. For a critical concentration of implanted ions it is possible to show using statistical arguments that conducting chains are formed by the union of these links, according to a percolation mechanism. We show that the assumption of a distribution of isolated implanted ions at low temperature is justified by the observation of their precipitation when the crystal is warmed up to room temperature. The transformation of the metallic optical absorption into the colloidal band associated with precipitates of implanted ions is correlated with the transition from a conducting state to an insulating state of the implanted layer. We show that this evolution towards an equilibrium situation may be reversed by a reirradiation, which is used as an external perturbation and that the conducting state associated with dispersed implanted ions is then once again obtained.