First-principles investigation on the geometries, stabilities and defective properties of fluoride surfaces

Abstract

From first-principles calculations, we perform a systematic study of the stoichiometric surface morphology of NaF, MgF2 and CaF2 and the associated stability, charge transfer and defective properties. Given the geometries of their low index surfaces, it is found that the surfaces with the lowest surface energies for NaF, MgF2, and CaF2 are (100), (110) and (111) surfaces, respectively. The dependence of surface energies, electrostatic potentials and effective charges on the slab thickness is discussed. Moreover, we demonstrate the broken bond model, which is based on the covalent interactions, is also suitable for ionic fluoride crystals after modification. By setting a fitting parameter k around 0.5, the estimated surface energies are close to the ones by slab modeling for all the 10 surfaces considered in this work.