The structure of NaCl(100) and KCl(100) single crystal surfaces: a tensor low energy electron diffraction analysis

Relaxation and rumpling of the (100) surfaces of NaCl and KCl single crystals were investigated by means of low energy electron diffraction (LEED). For primary currents in the range of 5 nA, sharp diffraction patterns of (1 x 1) symmetry were measured in the temperature range between 20 and 295 K. Under these conditions, neither charging effects nor electron-induced surface damage were observed. The temperature dependence of the peak intensities is described by a surface Debye temperature of 174 +/- 3 K for NaCl and 146 +/- 10 K for KCl, respectively, From the sequence of diffraction patterns measured in the energy range between about 60 and 300 eV, intensity versus electron energy curves (I(P) curves) were obtained for several diffraction peaks, and analyzed with the tensor LEED approach. Structural as well as non-structural parameters were varied systematically. According to the LEED analysis, the distance between first and second layer of NaCl is contracted by 1.4(7)% relative to the bulk value, and the chloride ions in the first layer are lifted by 0.07(3) Angstrom while the sodium ions are shifted towards the bulk by the same amount. The second and third layer show almost no rumpling and layer distances close to the bulk value. For KCI, the spacing between the top layers is identical to the bulk value within the error bars. The cations of the first layer are relaxed towards the bulk by only 0.03(5) Angstrom and the anions by the same amount outwards. For both substrates, the refined final structures were characterized by Pendry R-factors of 0.13. (C) 2001 Elsevier Science B.V. All rights reserved.