Order-Disorder Transition of the W(001) Surface.

Synchrotron x-ray diffraction has been used to study the surface reconstruction of W(001) through its phase transition at around 230K. We find that, although the superlattice peak intensity decreases 1000 fold 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.