Symmetry breaking and atomic displacements in the 3 x 3 surface phase of Pb/Ge(111)

The atomic structure of the low-temperature 3 x 3 surface phase of Pb/Ge(1 1 1) has been analyzed using the surface X-ray diffraction technique. The a tomic displacements found in this phase give rise to a structure distorted both in the surface plane and in the vertical direction. This structure is compared with the root 3 x root 3R30 degrees parent phase found at room tem perature. Several of the symmetry elements present in the ideal, flat root 3 x root 3R30 degrees phase, are absent in the 3 x 3 phase. The origin and extent of the symmetry breaking at low temperature is discussed in detail. (C) 2000 Elsevier Science B.V. All rights reserved.