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

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 Angstrom for the films with KCl thicknesses of 25, 6 and 2 Angstrom, respectively. The first one of 3.13 Angstrom for the 25 Angstrom 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 K. 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 A film shows an anisotropy of the K-Cl distance (3.16 and 3.13 A for the in-plane and out-of-plane distances, respectively), this resulting in the face-centered tetragonal structure of the KCl film, (C) 1999 Elsevier Science B.V. All rights reserved.