We have investigated the structures of Cu(1 1 1)(root3 x root3)R30degrees-Sb using time of flight-impact collision ion-scattering spectroscopy. The experimental data and computer simulations support a structural model for the Cu(1 1 1)(root3 x root3)R30degrees-Sb structure in which Sb atoms displace up to 1/3 of the first layer of Cu atoms and incorporate them into the first Cu layer with the Sb atoms displaced outward 0.40 Angstrom with respect to the first-layer Cu atoms. The outermost first layer of Sb and Cu atoms shift from fee- to hop-hollow sites (only the top layer of Sb and Cu atoms occupies hop hollow sites). (C) 2002 Elsevier Science B.V. All rights reserved.