We present a comparative study of MgO and NiO ultrathin films on Ag(001) by polarization dependent X-ray absorption spectroscopy on both the metal and oxygen K-edges. The data have been analyzed using ab initio multiple-scattering calculations (FEFF code). We show that the films have the rock-salt structure with a negligible atomic interdiffusion with the substrate in all cases. Both MgO and NiO films are tetragonally strained to match the Ag substrate at low thickness and gradually relax their structure to the bulk one. The local in-plane and out-of-plane interatomic distances in the oxides have been accurately determined as a function of thickness and the elastic constants of the films have been found to be compatible with the bulk values. The oxygen absorption site has been found to be on top of Ag atoms, while the metal occupies the interstitial site in both systems. A significant increase of the interplanar distance at the interface between both MgO and NiO and the Ag substrate has been measured.
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