Structure and electrocompression of chloride adlayers on Cu(111)

► We revealed the Cl/Cu(111) adlayer structure by in-situ surface X-ray diffraction. ► An electrocompressible 2D incommensurate adlayer is found for Cl/Cu(111). ► The strong adsorbate-adsorbate interaction disagrees with the Novaco-McTague model. ► The orientational epitaxy follows the Grey-Bohr model based on symmetry considerations. ► The ionic bonding and the remaining charge of the ad-atom control the adlayer structure.