Order-disorder character of the (3x3) to (sqrt3xsqrt3)R30 phase transition of Sn onGe(111)

The α phase of Sn/Ge(111) has been investigated from 120 K up to 500 K, using synchrotron radiation core-level photoemission. By means of photoelectron diffraction experiments, we verified that the rippled structure of the low-temperature (3×3) phase is preserved in the (√3×√3)R30° phase at room temperature, thus confirming the order-disorder character of the phase transition. We also found that at least two components are present in the Sn 4d core-level spectra up to 500 K, i.e., about 300 K above the onset of the transition from the low-temperature (3×3) phase to the (√3×√3)R30° phase, thus excluding the occurrence of any displacive transition.