  The ground-state structures and ferroelectric properties of NbN (N=2-52) have
been investigated by a combination of density-functional theory (DFT) in the
generalized gradient approximation (GGA) and an unbiased global search with the
guided simulated annealing. It is found that the electric dipole moment (EDM)
exists in the most of NbN and varies considerably with their sizes. And the
larger NbN (N>=25) prefer the amorphous packing. Most importantly, our
numerical EDM values of NbN (N>=38) exhibit an extraordinary even-odd
oscillation, which is well consistent with the experimental observation,
showing a close relationship with the geometrical structures of NbN. Finally,
an inverse coordination number (ICN) function is proposed to account for the
structural relation of the EDM values, especially their even-odd oscillations
starting from Nb38.
