The Surface Energies of Inert-gas and Ionic Crystals

It is shown that the surface energies of inert-gas crystals are proportional to their sublimation energies. The constant of proportionality is evaluated for the {111},- {100} and {110} faces for different assumed forms of the interatomic repulsive forces; to within 15% it is independent of the face, and of the form of the repulsive force. The distance of the outermost plane of atoms from the next at a {100} face is 2.54% greaterthan normal; this distortion makes a negative contribution of 1% to the surface energy. The surface energies of sodium and potassium halide crystals are calculated; the ratio-of the energies of {110} and {100} faces is about 2.5. The contribution of van der Waals-type forces to the surface energy of a {100} face is almost half the total. Elementary methods are given for the evaluation of lattice sums.