Etch-Tunnels in Lithium Fluoride Crystals

Abstract

The dissolution and growth behavior of lithium fluoride crystals in aqueous solutions containing long-chain fatty acids as step-poisons has been investigated. Both positive crystals (whiskers and platelets) and negative crystals (etch-tunnels) can be grown, respectively, from supersaturated and undersaturated solutions of lithium fluoride containing identical concentrations of stearic acid (∼2×10−6N). Dissolution behavior in stagnant environments is of particular interest for etch-tunnels some 0.5 μ in diameter and more than 100 μ in length (negative whiskers) have been observed. Two types of etch-tunnel can be distinguished; those nucleated at a surface defect—such as a cleavage step or the point of emergence of a dislocation (D type)—and those nucleated at apparently random points on the crystal surface (R type). Metallographic observations reveal that dislocations are not essential to negative crystal growth and it is proposed that tunnels grow simply because dissolution is less efficiently inhibited at the end of a tunnel than at the external surface of the crystal. This is a consequence of the reduced number of poison molecules available to inhibit kink and step motion at the end of a tunnel because of their slow rate of diffusion in a stagnant environment. Vigorous agitation of the environment suppresses etch-tunnel formation.