Using X-ray standing waves, the vertical position of Br ions of a thin
epitaxially grown overlayer of LiBr on a Si(111) substrate has been d
etermined. This system is a prototype for the epitaxial growth of high
ly ionic compounds onto covalent semiconductors. The results are consi
stent with a polar LiBr(111) bilayer, with Br ions in the lower level
in on-top sites above the Si interface and Li ions occupying the upper
level.