  Nanocrystalline Ni can adopt a hexagonal close-packed (hcp) structure,which
does not appear in a bulk form under any conditions.Structural studies found
its $>$ 20% expansion of volume per atom compared with face-centered cubic
(fcc) Ni.Hard x-ray photoemission spectroscopy of the hcp phase revealed a $>$
0.14 valence electron transfer from 3$d$ to 4$s$ and 4$p$ orbitals and a 0.25
eV reduction in band energy per electron compared with the fcc phase.Its
nonmagnetic nature was attributed to a low density of states at the Fermi
level.Thermodynamic stability of the hcp phase can be explained by the band
energy gain and the electrostatic potential energy loss as related to the large
lattice expansion.
