The structure and phase behavior of the clean Pt(111) surface has been studied between 300 K and 0.92${\mathit{T}}_{\mathit{m}}$ via x-ray scattering. The surface is unreconstructed at low temperatures (T<0.65${\mathit{T}}_{\mathit{m}}$). For temperatures greater than ${\mathit{T}}_{\mathit{c}}$=0.65${\mathit{T}}_{\mathit{m}}$, we find that the top layer undergoes a continuous commensurate-incommensurate transformation into a translationally and orientationally disordered discommensuration-fluid phase, which is isotropically compressed relative to the bulk (111) planes. A disordered arrangement of discommensurations separates regions of ideal face-centered-cubic ABC stacking from regions of faulted ABA stacking. As the temperature is increased, the compression of the surface layer increases and the incommensurability (δ) follows a power-law versus reduced temperature with an exponent equal to 1/3. For temperatures increasing above 0.75${\mathit{T}}_{\mathit{m}}$, sixfold orientational ordering of the discommensurations develops and increases.

• Received 4 August 1993

DOI:https://doi.org/10.1103/PhysRevB.48.18119