Order-Disorder Transition of the W(001) Surface

摘要

Synchrotron x-ray diffraction has been used to study the surface reconstruction of W(001) through its phase transition at around 230 K. We find that , although the superlattice peak intensity decreases a thousandfold over the range studied, the integrated intensity, and hence the structure factor, is nearly constant. This indicates that the surface undergoes an order-disorder transition with little change in the magnitude of the atomic displacements. The critical behavior is compared with predictions of the 2D XY and Ising models. The belief that there is a change of structure associated with this phase transition rests almost entirely on the LEED reports of a large drop in intensity of the half order diffraction beams above 200 K. In this paper we wish to qualify this basic experimental fact, by reporting that the kinematical x-ray diffraction intensity is nearly constant throughout the entire temperature region 130-360 K and that above the transition temperature T the peak merely changes shape to conserve its volume. Since there is no change in structure factor, the local structure is unaltered through the transition and so the HT phase must be disordered. by measuring the peak widths as function of T, we find that as the ordered domains diminish in size they retain an anisotropic 2:1 axial ratio. The peak widths scale approximately linearly with T-T, consistent with the classification of the phase transition as a 2D XY model with strong cubic anisotropy. Molecular structure. Reprints. (jes)