Stabilization of Nonmagnetic Hexagonal Close-Packed Phase in Nanocrystalline Nickel Metal with a Giant Volume Expansion

TL;DR

This study demonstrates the stabilization of a nonmagnetic hexagonal close-packed phase in nanocrystalline nickel, characterized by significant volume expansion and electronic structure changes, revealing new insights into phase stability at the nanoscale.

Contribution

It provides the first detailed analysis of the electronic and thermodynamic factors stabilizing the hcp phase in nanocrystalline nickel.

Findings

> 20% volume expansion in hcp Ni compared to fcc Ni

Valence electron transfer from 3d to 4s/4p orbitals

Nonmagnetic behavior due to low density of states at Fermi level

Abstract

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.