Order-disorder character of the (3 x 3) to (sqrt3 x sqrt3)R30° phase transition of Sn on Ge(111)

The a 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 x 3) phase is preserved in the (root3 x root3)R30 degrees 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 x 3) phase to the (root3 x root3)R30 degrees phase, thus excluding the occurrence of any displacive transition.