  The Ge(001)-(2 1 0 3)-Pb surface reconstruction with a lead coverage of 5/3
monolayer is on the borderline between the low-coverage covalently-bonded and
high-coverage metallic lead overlayers. This gives rise to an unusual
low-temperature phase transition with concomitant changes in the bonding
configuration. Both the room-temperature and low-temperature phases of this
system were investigated by surface x-ray diffraction using synchrotron
radiation. The room-temperature Ge(001)-(2 1 0 3) phase is best described by a
model with dynamically flipping germanium dimers underneath a distorted Pb(111)
overlayer with predominantly metallic properties. In the low-temperature
Ge(001)-(2 1 0 6) phase the dimers are static and the interaction between
adsorbate and substrate and within the adsorbate is stronger than at room
temperature. These results suggest that the phase transition is of
order-disorder type.
