Equilibrium shape of NaCl crystals: A first-principles calculation

It has recently been observed that the shape of NaCl crystals in stable coexistence with the vapor, while strictly cubical at low temperature, undergoes a corner-rounding transition at T₀{}650 ^∘{}C. A first-principles theory of the thermal evolution of the NaCl crystal shape is presented. The theory takes proper account of the statistical mechanics of atomic-scale interface fluctuations and provides a framework for understanding the qualitative differences observed between the thermal evolution of NaCl and that of metal crystals. Corner rounding occurs for NaCl via an energy-entropy crossover mechanism. The role of steplike excitations in building vicinal surfaces is particularly emphasized. Estimates of T₀ depend directly and sensitively on the energy per unit length of steps. Our calculations lead to a value of T₀ about a factor of 2 too high. We attribute this discrepancy to the neglect of atomic relaxation effects at and near surface steps.