Phase Transitions in clean Metal Surfaces.

As temperature is changed, two entirely different kinds of structural phase transition can be seen in surfaces. The first involves reconstructed surfaces which can transform to a new structure or to a disordered state. We present x-ray data which show that W(001) undergoes such an order-disorder transition. The second class is roughening transitions from smooth at low T to rough at high T; N(113) shows this behavior. Lastly we consider the interesting case of Pt(110) which appears to have characteristics of both kinds.