**Quantification
of substitutional disorder and atomic vibrations by LEED - the role of
parameter correlations**

**V. Blum, L. Hammer, W. Meier and K. Heinz**

Lehrstuhl für Festkörperphysik, Universität Erlangen-Nürnberg,
Staudtstrasse 7, D-91058 Erlangen, Germany
##
Surface Science 488 (2001) 219-232

The composition of substitutionally disordered surfaces, including the
layer and site dependent distribution of the chemical elements involved,
has frequently been a target of quantitative low-energy electron diffraction.
In the present paper, we demonstrate that, at a certain precision level
of the intensity analysis, the structural search for these chemical parameters
suffers from their correlations with statistical atomic displacements from
the ideal lattice sites. The latter can be due to thermal vibrations and/or
induced by the random substitution of atoms. Using the example of two (1
0 0)-oriented Fe_{1-x}Al_{x} alloy surfaces
(*x*=0.15 and 0.47), we show that unusually large error limits for
chemical parameters result when such atomic displacements must be determined
simultaneously. Even small systematic errors in the intensity analysis
can considerably shift the best-fit chemical structure. In order to avoid
incorrect conclusions, the parameter correlations should be taken into
account explicitly.