The structure of CaF2(1 1 1) and BaF2(1 1 1) single crystal surfaces: A tensor low energy electron diffraction study

The surface structures of the wide band-gap insulators CaF2(1 1 1) and BaF2(1 1 1) were investigated by means of low energy electron diffraction (LEED) with primary beam currents in the nA range. Beam intensities were recorded at 20 K sample temperature between 80 and 360 eV and analyzed using the tensor LEED approach. The results provide for the first time quantitative experimental information about the relaxation of these surfaces. The CaF2(1 1 1) surface deviates only insignificantly from a truncated fluorite bulk lattice with F–M–F layer structure (M: Ca or Ba), the largest displacement being the inward shift of the topmost layer of fluoride anions by 0.03 ± 0.06 Å. The same contraction of the interlayer distance at the interface is significant for the BaF2(1 1 1) surface, for which the topmost anion is displaced towards the bulk by 0.12 ± 0.07 Å. The positions of the alkaline earth metal cations as well as those of the second fluoride layer in this topmost F–M–F sheet are almost identical to their bulk values. The same holds for all deeper layers. From the temperature dependency of the diffraction intensities surface Debye temperatures ΘD,surface of 196 ± 19 K (CaF2) and 125 ± 6 K (BaF2) were deduced, respectively. The results suggest that the enhanced thermal motions are restricted to the topmost F–M–F layer only.