PNNL

Abstract

Bond paths and the bond critical point properties (the electron density, ?, and the Hessian of ? at the bond critical point) have been calculated for the bonded interactions comprising the Ni–sulfide minerals millerite, vaesite and heazlewoodite and Ni metal. The experimental NiS bond lengths decrease linearly as the magnitudes of the properties at the bond critical point, bcp, increase in value. NiNi bond paths exist between the Ni atoms in heazlewoodite and millerite for NiNi separations that match the shortest NiNi separation in Ni metal, an indicator that the Ni atoms are bonded. NiNi bond paths also exist between the Ni atoms in bulk Ni metal. The bcp properties of the NiNi bonded interactions in Ni metal are virtually the same as those in the two Ni sulfides. NiNi bond paths are absent in vaesite where the NiNi separations are 60% longer than those in Ni metal. The bcp properties for the NiNi bonded interactions scatter along protractions of the NiS bond length–bcp property trends, suggesting that the two bonded interactions have similar characteristics. The NiNi bond paths radiate throughout Ni metal and the metallic heazlewoodite structures as continuous networks of interconnected NiNi bond paths whereas the NiNi paths in millerite, a highly covalent d,p metal, are restricted to isolated Ni3 rings of NiNi bond paths interconnected by S atoms. Electron transport in Ni metal and heazlewoodite is pictured as occurring along the bond paths, which behave as networks of atomic size wires that radiate in a contiguous circuit throughout the two structures. Unlike heazlewoodite, the electron transport in millerite is pictured as involving a cooperative hopping of the d–orbital electrons from the Ni3 rings comprising Ni3S9 clusters to Ni3 rings in adjacent clusters via the p–orbitals on the interconnecting S atoms. Vaesite, an insulator at low temperatures and a doped semiconductor at higher temperatures, lacks NiNi bond paths. The net charges conferred on the Ni and S atoms are about a quarter that of their nominal formal valences for the atoms in millerite and vaesite with the net charge on Ni increasing with increasing NiS bond length. The reduced net charge on the Ni atom in heazlewoodite is related to its NiNi metal bonded interactions and the contiguity of the NiNi bond paths.