  Growing attention has been drawn in the past years to the \alpha-phase
  (1/3 monolayer) of Sn on Ge(111), which undergoes a transition from the low
temperature (3x3) phase to the room temperature (\sqrt3 x \sqrt3)R30&#176; one.
On the basis of scanning tunnelling microscopy experiments, this transition was
claimed to be the manifestation of a surface charge density wave (SCDW), i.e. a
periodic redistribution of charge, possibly accompanied by a periodic lattice
distortion, which determines a change of the surface symmetry.
  Recent He diffraction studies of the (3x3) long range order have shown the
transition to be of the order-disorder type with a critical temperature Tc=220
K and belonging to the 3-state Potts' universality class. These findings
clearly exclude an SCDW driven mechanism at 220 K, but they cannot exclude the
occurence of a displacive transition at higher temperature. Here we present
photoelectron diffraction data taken at 300 K and photoemission data taken up
to 500 K (which is the maximum temperature where the (\sqrt3 x \sqrt3)R30&#176;
is stable) . From our analysis it is shown that the atomic structure of the Sn
overlayer does not change throughout the transition up to 500 K. As a
consequence the displacive hypothesis must be discarded in favour of a genuine
order-disorder model.
