Origin of anomalous surface lattice expansion in Pd(001)

We present a systematic study of the hydrogen induced anomalous expansion of the top layer spacing, d(12), using low-energy electron diffraction and surface x-ray. scattering. After exposure of 6 L of hydrogen at 150 K sample temperature, the hydrogen atoms fully occupy the surface hollow sites and a lattice expansion of Delta d(12)=+4.7% is determined in agreement with theoretical predictions (5.2%). Heating the sample above the hydrogen desorption temperature (T-D approximate to 340 K), leads to an almost complete relaxation of d(12) to the bulk value of 1.945 angstrom. Similarly, no expansion is observed for clean Pd(001) prepared by rapid cooling after quick thermal treatment to remove hydrogen. Hydrogen re-adsorption from the residual gas atmosphere and possibly hydrogen agglomeration in the near surface region leads to an expansion of d(12) in the 2%-3% range as observed in previous experiments. On the basis of our results, surface magnetism as a mechanism to trigger lattice expansion in Pd(001) does not need to be invoked.