On the Colloid Evolution in Alkali Halide Crystals: A Proposed Tentative Mechanism

Abstract

An investigation of photo and thermochemical reactions in additively colored and subsequently annealed (at 550°C) single crystals of ‘Pure’ Optovac NaCl crystals has been made and utilising the colloid evolution as a criterion our crystals could be classified as: (1) Those which revealed the colloids and those which did not; (2) Those which revealed the colloids and also exhibited the coexistence of F centers and colloids above about 250°C.
Prolonged annealing revealed that the colloids are stable up to about 250°C and that they became unstable above this temperature. Our results in this regard are compared with the results of Doyle on KCl.
It is proposed that the diffusion of neutral F centers via the vacancy mechanism results in their clustering and when the cluster attains a critical size the F center electrons enter the valence levels of the alkali ions to form metal atoms of the colloids. Our results, as well as the results in the literature, are discussed in the light of our tentative mechanism.