Theoretical study of the adsorption of a CaF2 molecule at the (1 1 1) surface of CaF2 I. Equilibrium adsorption positions

The present paper is concerned with theoretical calculations of the adsorption positions and energies of a CaF2 molecule at different sites (terrace, step and kink) on the (1 1 1) surface of a calcium fluoride crystal. Calculations are performed using pair potentials considering relaxation of crystal ions in a defined vicinity of the admolecule. In addition to earlier determined relaxations without adsorption of the terrace- and the [1 0 1̄]-type I step-sites also the configuration of the kink at such a step is determined. For a CaF2 molecule the minimum adsorption energies of −1.73, −3.58 and −5.05 eV at the terrace, step, and kink positions, respectively, are found. The latter value is in satisfactory agreement with the experimental third law value of the sublimation energy of −4.48±0.09 eV at 298 K as well as with the lattice energy per molecule. Results for adsorption at the terrace can be used to understand, by analogy, the experimental observation of the emission of MeX3− ions (Me: Ba2+, Sr2+; X: F−, Cl−) from (1 1 1) surfaces of BaF2 and SrCl2, respectively. The calculation of the kink density in equilibrium leads to nk=5.18×107exp(−5.487×103/T) cm−1 with a formation energy of 0.47 eV. This corresponds to a mean distance of kinks at the [1 0 1̄]-I step of about 40 nm at 1000 K.