  A stress exists in solids surfaces, similarly to liquids, also if the
underlying bulk material is stress-free. This paper investigates the surface
stress effect on the measured value of the Si lattice parameter used to
determine the Avogadro constant by counting Si atoms. An elastic-film model has
been used to provide a surface load in a finite element analysis of the lattice
strain of the x-ray interferometer crystal used to measure the lattice
parameter. Eventually, an experiment is proposed to work a lattice parameter
measurement out so that there is a visible effect of the surface stress.
