Dislocation Etch Pit Formation in Sodium Chloride

Abstract

Dislocation etch pits can be formed on (100) surfaces of NalC crystals by etching in a solution of FeCl₃ in glacial acetic acid. Observations and arguments are enumerated to illustrate the reliability of the etching technique to reveal dislocations at positions where they intersect the crystal surface. The difference in shape of etch pits at edge and screw dislocations is consistent with that expected due to their different angles of inclination. Etch pit formation depends on variables of the etching solution and procedure. Insufficient FeCl₃ in solution results in wide, indistinctly shaped pits, whereas excessive FeCl₃ results in small pits with rounded corners. NaCl in the solution decreases the etching rate, while the presence of water increases the etching rate and results in more shallow pits. The added salt in the solution is believed to act as an inhibitor which poisons the dissolution process in the same manner as that proposed by Gilman et al. [J. J. Gilman, W. G. Johnston, and G. W. Sears, J. Appl. Phys. 29, 747 (1958)] for LiF. Agitation increases the size of the pits for a given etching time.