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.