A 7x7-2x7 Phase transition of Au overlayers on Ni(110) surface.

When a Ni - 0.8% Au alloy is annealed at temperatures above 400C, Au segregates at the Ni(100) surface to form an overlayer providing 1.0 +- 0.05 monolayer coverage. Low energy electron diffraction (LEED) showed that the surface periodicity is 7x7 at room temperature and 2x7 at temperatures above 340 +- 10C. The 7x7-2x7 transition was observed with a novel LEED system employing a position-sensitive detector to make digital records of LEED patterns. The transition was found to be abrupt and to display marked hysteresis. The results have been interpreted as follows on the basis of structural models of the overlayer inferred from LEED and low energy ion scattering observations on the 7x7 surface.