The superstructures of the clean Pt(100) and Ir(100) surfaces

The clean and reconstructed surfaces of Pt(100) and Ir(100) were investigated by low energy electron diffraction (LEED). It is shown that two superstructures can be observed in the case of platinum. The structure Pt(100)-hex, which is commonly called Pt(100)-(5 × 20), transforms to Pt(100)-hex-R0.7° above 1100 K. It is shown that this stable phase differs from the first one by a slight rotation of the hexagonal surface layer by 0.7°. For Ir(100) only the well known (1 × 5) superstructure is observed without any rotation of the outer layer. The rotation angle of 0.7° for platinum and the stability of the unrelated structure for iridium can be interpreted by simple calculations of the coordination of surface atoms with those of the second layer. The method assumes that the surface layer is of ideal hexagonal structure in the case of platinum and nearly hexagonal in the case of iridium. The results are in good agreement with the experiment.