Role of Ca⁺⁺ impurities in the evaporation kinetics of NaCl and KCl crystals

The vacuum evaporation of NaCl and KCl crystals doped with 300 ppm calcium impurities is studied. The gold decoration technique is used to develop monotomic steps on the evaporating surfaces. It is found that impurities tend to diffuse from the voluem to the crystal surface forming a second phase on ti. The impurity nuclei interact with ledges producing several step reactions. The ratio of the perpendicular components of step reactions. The ratio of the perpendicular components of step velocities in 〈100〉 and 〈100〉 surface directions has been measured as a function of the heating period. The calcium impurities tend to reduce this ratio from 1.0 to a limiting value of 0.72. It is concluded that divalent impurities play a dominant role on the topography of the evaporating surface.