RECONSTRUCTION OF THE PT(111) SURFACE - X-RAY-SCATTERING MEASUREMENTS

The structure and phase behavior of the clean Pt(lll) surface has been studied between 300 K and 0.92T(m) via x-ray scattering. The surface is unreconstructed at low temperatures (T<0.65T(m)). For temperatures greater than T-c=0.65T(m), we find that the top layer undergoes a cont inuous commensurate-incommensurate transformation into a translational ly and orientationally disordered discommensuration-fluid phase, which is isotropically compressed relative to the bulk(lll) planes. A disor dered arrangement of discommensurations separates regions of ideal fac e-centered-cubic ABC stacking from regions of faulted ABA stacking. As the temperature is increased, the compression of the surface layer in creases and the incommensurability (delta) follows a power-law versus reduced temperature with an exponent equal to 1/3. For temperatures in creasing above 0.75T(m), sixfold orientational ordering of the discomm ensurations develops and increases.