Interface structure of alkali-halide heteroepitaxial films studied by x-ray-absorption fine structure

We have investigated the interface structures of KCl and NaCl thin films grown on NaBr(100) by means of Cl K-edge x-ray-absorption fine structure. The x-ray-absorption near-edge structure of 1-ML-thick KCl on NaBr indicates that the mixed crystal does not form at the interface. Through the analysis of extended x-ray-absorption fine structure, it was revealed that with a decrease in film thickness, the K-Cl bond distance is slightly shortened in KCl/NaBr(100), while the Na-Cl bond is elongated in NaCl/NaBr(100). The 1-ML-thick film does not, however, form coherent bonds with the substrate. This implies that the in-plane bond distance of the film does not coincide with that of the substrate but is close to the bond distance of the bulk material. For further understanding of the bond character between the film and substrate, Monte Carlo calculations have been carried out for the KCl/KBr system which gave a similar conclusion in our previous study. The calculated results were found to agree with the experimental results reasonably. These findings are contradictory with the previous diffraction results that have suggested the formation of coherent bonding at the interface. Such a discrepancy implies that the short-range order (bond distance) does not always reflect the long-range order (lattice constant).