Thermal etching of (100) surfaces in sodium chloride single crystals

Freshly cleaved sodium chloride single crystals thermally etched both in air and in vacuum (10−5 torr) show three types of evaporation pit: concentric square pits and concentric circular pits in air etched crystals, and pyramidal pits in vacuum etched crystals. The pyramidal pits may be of three kinds: those with symmetric diagonals, those with one symmetric and one non-symmetric diagonal, and finally those with two non-symmetric diagonals. These pyramidal pits are of the same nature as those associated with dislocations, found on chemical etching of LiF crystals. The differences between pits produced in air and pits produced in vacuum are discussed in terms of surface fusion.