UHV TRANSMISSION ELECTRON-MICROSCOPY OF IR(001) .2. ATOMIC POSITIONS OF THE (5X1) RECONSTRUCTED SURFACE FROM HREM AND R-FACTOR REFINEMENTS

A partial solution of the atomic scale structure of the Ir(001)-(5 x 1) reconstructed surface is reported combining direct high resolution electron microscopy and R-factor analyses of transmission electron diffraction data. From the high resolution imaging, the structure can be uniquely identified as a hexagonal monolayer on the surface with at most small, distortions. R-factor minimization of the near surface structure based upon 8-bit digitization of the diffraction data shows systematic differences for models using single and multiple layers, demonstrating the existence of sub-surface strain fields. Unfortunately, the intrinsic error of 8-bit digital data is too large for a full multilayer minimization. Fitting was also performed using sub-surface relaxations constrained to match analytical strain solutions. These models indicate a two-fold bridge registry of the surface hexagonal layer with a three-fold hollow registry of the sub-surface atoms with respect to the surface layer and small sub-surface relaxations. The data reduction is quantitative with a 20-30% coverage, in agreement with experimental imaging data, and the accuracy of the atomic positions is about +/- 0.005 nm.