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The 6-state model is proposed to describe the hydrogen-induced √3×√3 (trimers) → 2×1 (zigzag chains) reconstruction of 1 monolayer of Sb atoms on Si(111). Attractive nearest neighbour Sb-Sb pair interactions and also triple interaction vt are considered. The phase diagram of the system with and without vt is calculated when no hydrogen atoms are adsorbed. Second order phase transitions are obtained between disordered phase and both ordered phases, √3×√3 and 2×1, when vt=0. The possibility of phase separation close the √3×√3 - 2×1 phase boundary is discussed when vt interaction is included. Reinterpretation of experimental results using our model provides us with a rough estimate of the structural phase transition temperature in the range 730 - 870 K. Corroborating recent experimental data, we demonstrate that adsorption of small coverage of hydrogen atoms turns the √3×√3 phase into mixed √3×√3 and 2×1 structure. The length and number of zigzag chains grows with increase of hydrogen concentration. |