During the study of Sb condensation on the Si(1 1 1)-In(root3 x root3) surface phase we observed the formation of a new Sb-induced surface structure with (root7 x root7) lattice below 450 degreesC. This phase may not be prepared by direct Sb deposition on the Si(1 1 1) surface. Instead, the substitution for In atoms from T-4 bonding sites by incoming Sb and formation of a gamma-phase strongly determine the Sb(root7 x root7) formation process. When the concentration of Sb exceeds the value of 0.2 ML the irreversible gammaroot3 --> root7 phase transition occurs. Based on STM observations, the structural model for this reconstruction has been proposed. (C) 2003 Elsevier Science B.V. All rights reserved.