A 7 × 7 → 2 × 7 Phase Transition of Au Overlayers on Ni(110) Surface

Abstract

AbstractWhen a Ni‐0.8% Au alloy is annealed at temperatures above 400 °C, Au segregates at the Ni(110) surface to form an overlayer providing 1.0 ± 0.05 monolayer coverage [1]. Low energy electron diffraction (LEED) showed that the surface periodicity is 7 × 7 at room temperature and 2 × 7 at temperatures above 340 ± 10°C. The 7 × 7 → 2 × 7 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 7 × 7 surface [1]. Most os the Au atoms in the 7 × 7 overlayer form close‐packed zigzag chains along the [110] semi‐channels provided by the Ni(110) surface. Sets of three Au atoms from adjacent semi‐channels combine in hexagonal formations, each of which is capped by an extra Au atom. The mismatch stresses that would result from unlimited extension of this close‐packed Au‐atom arrangement are relieved by interruptions of the zig‐zag sequences to form antiphase boundaries parallel to [001]. The 2 × 7 periodicity results if the zig‐zag sequences are uninterrupted. The 7 × 7 → 2 × 7 transition involves the translation of alternate rows of Au atoms in the [001] and [001] directions by about half the Au‐Au neighbor distance. The geometry is such that the transition could be driven by a small lateral expansion of the Au overlayer relative to the Ni(110) surface.