The (2 × 1) structure of the two-dimensional nickel silicide surface alloy on Ni(111) was investigated using quantitative low-energy electron diffraction analysis. The unit cell of the determined silicide structure contains one Si and one Ni atom, corresponding to a chemical formula of NiSi. The Si atoms adopt substitutional face-centered cubic hollow sites on the Ni(111) substrate. The Ni-Si bond lengths were determined to be 2.37 and 2.34 Å. Both the alloy surface and the underlying first layers of Ni atoms exhibit slight corrugation. The Ni-Si interlayer distance is smaller than the Ni-Ni interlayer distance, which indicates that Si atoms and underlying Ni atoms strongly interact.
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