The development of thermal etch pits on cleaved NaCl crystals in the presence of an electric field

Abstract

The development of thermal etch pits on cleaved (100) surfaces of sodium chloride crystals was investigated in relation to an external DC electric field applied along the [100] direction. Measurements of pit size and density were made after isothermal and isochronic evaporation in fields with gradients ranging up to 130 kV m -1. Below a minimum (threshold) value of field gradient, the size and density of pits were unaffected by the presence of the field. In the presence of a threshold field gradient, evaporation resulted in the enhancement of nucleation of pits as judged by a marked increase in their density with no change in their size. At higher fields, enhanced pit nucleation and pit growth are evident. The field gradient threshold observed in relation to thermal pit development corresponded to previously reported threshold phenomena for enhanced evaporation from cleaved (100) surfaces of NaCl. Measurements of dislocation densities in regions below the evaporated surface showed a decrease in density with increasing field strength beyond the threshold value. These results are interpreted as suggesting a migration of charged dislocations in the presence of the DC field.