Room temperature Si(001)-(2x1) reconstruction solved by x-ray diffraction

X-ray diffraction has been used to characterize the room-temperature atomic structure of the clean Si(001)-(2 x 1) reconstruction. intensities of 276 half-integer order reflections have been measured and, after considering the symmetry of the system, they reduced to 44. Different models have been used to try to fit the experimental data. The best fit structure clearly indicates the presence of asymmetric dimers (bond length 0.267 nm and tilt angle 20 degrees) with a disordered buckling direction. Atomic relaxations down to the sixth layer have been determined From the analysis of the Si-Si bond distances we deduce that this (2 x 1) structure is likely to be a disordered version of a higher-order reconstruction, p(2 x 2) or c(4 x 2).