ETCH-TUNNELS IN LITHIUM FLUORIDE CRYSTALS AND CONCERNING 'ANOMALOUS' SOLUTION AND ETCHING PHENOMENA

The dissolution and growth behavior of LiF crystals in aqueous solutions containing longchain fatty acids as step-poisons was investigated. Both positive-crystals (whiskers and platelets) and negative-crystals (etch- tunnels) can be grown respectively from supersaturated and undersaturated solutions of LiF containing identical concentrations of stearic acid (approximately 2 x 10 to the -6th power N.). Dissolution behavior in stagnant environments is of particular interest for etch-tunnels; some 0.5 microns in diameter and more than 100 microns in length (negative-whiskers) were 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 negativecrystal growth.