Theoretical study of the adsorption of a CaF2 molecule at the (1 1 1) surface of CaF2 II. Surface diffusion

This paper is concerned with a theoretical study of the diffusion pathways of a CaF2 molecule on the terrace of the (1 1 1) surface of a CaF2 crystal. Altogether, the surface diffusion can be described by a staggering movement, where the molecule swings in every jump around one of its fluorine ions being fixed to a calcium ion of the outermost surface triple layer of the crystal. The diffusion can include exchanges of fluorine ions between the admolecule and the outermost fluorine layer at the surface. The jump distance results as a0/2=3.86 Å and the activation energy for diffusion as 0.80±0.10 eV. Assuming a vibration frequency in the order of 1013 s−1 the surface diffusion coefficient becomes Ds=3.73×10−3exp(−9.280×103/T) cm2 s−1. With the desorption time from the terrace τdes=10−13exp(1.972×104/T) s with a mean adsorption energy of −1.70±0.03 eV the mean diffusion displacement on the terrace becomes xs=2Dsτdes=3.86×10−8exp(5.220×103/T) cm with a saddle point energy of Es=0.90±0.13 eV.