Regensburg 2010 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 26: Oxides and insulators: Clean surfaces

O 26.8: Vortrag

Dienstag, 23. März 2010, 12:15–12:30, H40

Relaxation and thermal vibrations at the NaF(100) surface — •Stephan Härtel, Jochen Vogt, and Helmut Weiß — Chemisches Institut der Universität Magdeburg, Universitätsplatz 2, 39106 Magdeburg, Germany

Various theoretical models predict that the (100) surfaces of ionic solids with the rocksalt structure should exhibit a surface rumpling, i. e. cations and anions in the surface layer have different vertical positions. While this type of relaxation has been measured for various alkali halides the rumpling of the NaF(100) surface is expected to be very small. We present results of an experimental determination of the relaxation of the NaF(100) surface based on low-current LEED experiments and a subsequent tensor-LEED analysis of diffraction peak intensities as a function of electron energy at various crystal temperatures. Moreover, we present experimental values of the mean amplitudes of the thermal motions of the surface ions. The experimental data are compared to the results of density functional theory (DFT) calculations using a slab model and ultrasoft pseudopotentials. According to both experiment and theory, the average vertical positions of the Na⁺ and F⁻ ions in the topmost surface layer differ at most by 0.02Å. While the measured rumpling is smaller than the error-bars, the model calculations predict a weak inward-displacement of the Na⁺ ions of 0.01Å, and an outward-shift of F⁻ of 0.01Å with respect to the truncated bulk geometry. The thermal motions of the ions in the topmost surface layer appear to be enhanced compared to the respective bulk mean amplitudes by a factor of 1.3 to 1.4.