Faceting and the orientational phase diagram of stepped Pt(001) surfaces

The results of synchrotron-x-ray-scattering studies of stepped Pt(001) surfaces are reported. An orientational phase diagram is proposed for Pt surfaces between the cubic (001) orientation and orientations tilted 6^∘{} away from the [001] direction towards the [110] direction, and for temperatures between 300 and 1850 K. Four distinct orientational phases are identified. At the highest temperatures, we find a one-phase region in which the surface is rough with the local surface normal equal to the macroscopic surface normal. Below T₀=1820 K, hexagonally reconstructed (001) facets appear in coexistence with the tilted rough phase. The phase boundary of the rough phase decreases with temperature as a quadratic function of the tilt angle away from the [001] direction, consistent with simple theoretical ideas. Below T₁=1630 K, there are two symmetry-related 2^∘{} magic vicinal phases with azimuthal orientations symmetrically rotated away from the cubic [110] direction by ±{}4.8^∘{}. The 2^∘{} magic vicinals are reconstructed, so that the terraces between their constituent steps form a corrugated and rotated quasihexagonal structure. Remarkably, the steps are parallel to the troughs and crests of the corrugation. In addition, the tilt angle appears closely related to the period of the reconstruction, favoring an integer (3 or 4) or half integer (9/2 or 7/2) number of corrugation periods on each terrace between steps, depending on the temperature. Finally, below T₂=1590 K, we find the 6^∘{} magic vicinal phase. Its microscopic structure involves terraces upon which there is one period of the surface corrugation.