When Pb is deposited on the Si(111) 7x7 surface at room temperature, a metastable two-dimensional (2d) phase results. Using grazing incidence synchrotron surface x-ray diffraction, we show that this phase is a commensurate 7x7 structure, but with a very different structure factor intensity distribution from the Si(111) 7x7. A good qualitative interpretation of the diffraction pattern is that Pb forms a 2d close-packed structure, with 8x8 Pb atoms per 7x7 Si unit cell. Upon annealing at 200°C, the metastable 7x7 phase transforms irreversibly into a stable phase that is incommensurate with the Si substrate. This incommensurate phase is also a 2d close-packed structure, but the Pb|1,0| lattice vector, instead of being parallel with Si |1,0| as in the 7x7 phase, is rotated 30°.
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Grey et al. (1989) studied this question.