The symmetry of Si(111)7x7.

The x-ray diffraction pattern of fractional order reflections from the 7x7 structure at a Si(111)/a-Si interface is found to have 6-fold symmetry, in contrast to the bulk. This supports the structural model of Takayanagi which has the same high symmetry. Upper limits are placed on the extent of displacements in both the reconstructed and lower, bulk-like layers of this structure.