Vacuum evaporation of NaCl and KCl crystal surfaces doped with divalent impurities

Thermal evaporation in vacuum from the surfaces of NaCl: Fe++; NaCl: Cd++; NaCl: Mn++ and KCl: Mn++ crystals has been studied. In all cases except that of cadmium, the impurity ions diffuse towards the surface and there produce nuclei of a second phase. These nuclei interact with the circular and rectangular step systems which are produced by evaporation and strongly modify the surface topography. The ratioV 〈110〉/V 〈100〉 of the perpendicular components of the step velocities has been measured as a function of heating time. On more prolonged heating the circular step structure is not stable, but becomes rectangular. This transition can be explained using the model proposed by Budke for vacuum evaporation.