We present a combined surface x-ray diffraction (SXRD) and theoretical analysis of the geometric structure of nanometer sized Co islands deposited on Cu(001) at 170 K. Two-dimensional nanoislands consisting of only 20–40 atoms are characterized by a 4%–8% contraction of the interatomic distances as compared to the bulk $\left(2.51\text{Å}\right)$. This strongly exceeds “usual” lattice relaxations normally seen for surfaces. The SXRD analysis is based on the analysis of the registry of the Co adatoms relative to the $\left(1\times 1\right)$ surface unit cell of the Cu(001) substrate crystal. Static displacements of the Co atoms of $0.18\text{Å}$ (root mean square) out of the equilibrium hollow sites are observed which are in agreement with predictions based on molecular-dynamics calculations.

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