Let us also report the following observations from : "For small particles the number of atoms close to the surface is comparable to that in the interior of the particle. The core-shell model of nanosize crystals assumes that in the core atoms are arranged in the crystallographic structure identical to that of bulk crystal, but the outer layers of the nanocrystals, the shells, although also arranged in the same crystallographic structure, have interatomic distances that are slightly increased (SiC, diamond) or decreased (GaN)" (see references given by Wieligor et al.). "The surface atoms may contribute less to the polarization than the core atoms because of the lack of counter-ions outside the grain" .