ORIENTATIONAL EPITAXY AND LATERAL STRUCTURE OF THE HEXAGONALLY RECONSTRUCTED PT(001) AND AU(001) SURFACES

We present results of synchrotron-x-ray-scattering studies of the orientational epitaxy exhibited by clean, hexagonally reconstructed Pt(001) and Au(001) surfaces. For the Pt(001) surface, a high-symmetry direction of the hexagonal overlayer is aligned with a high-symmetry direction of the bulk between 1820 and 1685 K. Between 1685 and approximately 1580 K, the relative rotation angle varies continuously from 0-degrees to 0.75-degrees with a one-half-power-law dependence on the reduced temperature. At approximately 1580 K, domains with a rotation angle of 0.8-degrees appear discontinuously, in coexistence with those with a rotation angle of 0.75-degrees. The two rotation angles reach approximately 0.9-degrees and approximately 0.75-degrees for the continuously and discontinuously rotated components, respectively, at 300 K. At all temperatures, the overlayer is incommensurate along both directions of the surface, with weakly temperature-dependent incommensurabilities. The areal density of the Pt(001) overlayer is compressed by approximately 8% with respect to the hexagonal (111) planes in the bulk. In addition, the extent of translational order within the surface layer decreases with decreasing temperature. The Au(001) surface exhibits a strongly discontinuous rotational transformation at approximately 980 K and there is coexistence between rotated and unrotated domains, in agreement with previous measurements. Rotated domains appear at a rotation angle of approximately 0.8-degrees and their number grows with decreasing temperature at the expense of unrotated domains. Our measurements reveal the existence of additional favored rotation angles: one of 0.9-degrees and one that varies smoothly from 0-degrees to 0.5-degrees. The relative domain populations depend on temperature. A mean-field theory of rotational transformations, which accounts for the continuous rotational behavior of the Pt(001) surface, is presented, and it is shown that there are no corrections to mean-field behavior from fluctuations for a rotational transformation.