  We analyze X-ray diffraction data used to extract cell parameters of
ultrathin films on closely matching substrates. We focus on epitaxial
La2/3Sr1/3MnO3 films grown on (001) SrTiO3 single crystalline substrates. It
will be shown that, due to extremely high structural similarity of film and
substrate, data analysis must explicitly consider the distinct phase of the
diffracted waves by substrate and films to extract reliable unit cell
parameters. The implications of this finding for the understanding of strain
effects in ultrathin films and interfaces will be underlined
