Heteroepitaxial growth of KCl on a cleaved (001) face of KBr studied by extended X-ray-absorption fine structure

Abstract

Thickness dependence of the local structure of epitaxially grown KCl films on KBr(001) was investigated by Cl K-edge EXAFS measurements at 150 K. The first-nearest neighbor KCl distance parallel to the surface was found to be elongated gradually with a decrease in the film thickness: 3.13, 3.16 and 3.17 Å for the films with KCl thicknesses of 25, 6 and 2 Å, respectively. The first one of 3.13 Å for the 25 Å film corresponds exactly to that of KCl bulk crystal. The present findings are contradictory to the previous diffraction results of the formation of coherent bonding at the KCl/KBr interface, which indicated that the KCl distance should be the bulk KBr distance of 3.28 Å. The present study revealed that even a very thin film is likely to maintain the natural KCl distance of bulk KCl, as in the case of solid solutions. The 6 Å film shows an anisotropy of the KCl distance (3.16 and 3.13 Å for the in-plane and out-of-plane distances, respectively), this resulting in the face-centered tetragonal structure of the KCl film.