X-ray absorption study at the Mg and O K-edges of ultrathin MgO epilayers on Ag(001)

We determined the local atomic structure of MgO epilayers on Ag(001) by means of polarization-dependent X-ray absorption spectroscopy measurements at the Mg and O K-edges. A quantitative analysis of the data in the extended energy range has been performed using multiple scattering simulations. We found that, even in the ultrathin limit, the local structure of the film is rock-salt and we obtained a quantitative evaluation of the average in-plane and out-of-plane film strain at the different thicknesses investigated. An in-plane compressive strain, due to lattice mismatch with the Ag substrate, is clearly present for the 3 ML film. The out-of-plane lattice constant is found to be expanded, in agreement with the expected behavior for a tetragonal distortion of the unit cell. This growth-induced strain is gradually released with increasing thickness and it is almost completely relaxed at 20 ML. Any significant intermixing with the Ag substrate can be ruled out. An expansion of the interplanar distance at the MgO-Ag interface is detected and its sign and magnitude are found to be in agreement with recent ab-initio simulations. This work provides previously unavailable input for modeling the physical properties of the system and supports the hypothesis that the different electronic properties of MgO films on Ag(001) are not related to structural or compositional differences at the ultrathin limit.