The atomic structure of thin epitaxial erbium suicides on Si(111) substrates has been studied in situ by means of surface X-ray diffraction and medium-energy ion scattering. In the sub-monolayer range, a two-dimensional (2D) suicide is formed within a 1 × 1 unit cell, with the Er atoms occupying T4
sites and a Si bilayer on top which is 180° rotated with respect to the bulk (5-type). Suicide layers with a thickness of 3 monolayers exhibit a regular network of Si vacancies which release the compressive strain of the graphite-like Si layers between adjacent Er layers. This results in a √3 × √3 R 30° unit cell, in which 3 out of 5 Si atoms are displaced towards the vacancy, and in which 3 Er atoms relax away from the vacancy.