Theoretical study of the adsorption of lithium fluoride molecules at the (111) surface of CaF2

The adsorption of different lithium fluoride molecules at the (111) surface of CaF2 was studied theoretically. Equilibrium adsorption positions and energies on the relaxed terrace and at the relaxed monomolecular 〈110〉 step of type I were calculated considering Coulomb, Born–Mayer and van der Waals interactions including an additional relaxation of crystal ions in the vicinity of the adsorbed molecules. The adsorption energies of the monomer LiF, the linear and the rhombic dimer Li2F2 on the terrace result as −1.72, −1.11, and −1.96 eV and as −2.68, −2.86, and −3.06 eV at the 〈110〉-I step. The monomer adsorption energy at the step is in good agreement with the value of −2.70±0.17 eV obtained recently by experimental investigations of the adsorption by desorption transients. Also, the experimentally observed dissociation of the lithium fluoride dimers into monomers at the steps can be understood on the basis of the present theoretical results. The linear dimer, which is metastable in the vapor but proved to be stable on the terrace and at the steps of CaF2(111), may be considered as the intermediate stage of the dimer dissociation.