ZnxNi1-xO rocksalt oxide surfaces:: Novel environment for Zn2+ and its effect on the NiO band structure

Rocksalt ZnxNi1-xO solid solutions, which form at the nickel-rich end of the phase diagram, can be prepared over a wide compositional range and are homogeneous UP to Zn0.32Ni0.68O when prepared by thermal dissolution in air. Auger parameter analysis indicates that the zinc is octahedrally coordinated, an unusual environment for solid-state Zn2+. The surface composition, as determined by Auger and X-ray photoelectron spectroscopies shows comparable surface and bulk concentrations until phase separation occurs, at which point the surface becomes enriched in wurtzite ZnO. The charge-transfer nature of the NiO electronic structure remains largely intact as zinc is added to the lattice. In ab initio Hartree-Fock calculations of the ZnxNi1-xO valence band, zinc interacts with the lattice oxygen primarily through the Zn 4s orbitals and is less covalent than in wurtzite ZnO. A detectable shift of O 2p character away from the valence band maximum to more strongly bonding energies accompanies increased zinc concentration as a result of Zn 4s hybridization with O 2p states. However, the zinc serves primarily to dilute nickel-nickel coupling interactions in the electronic structure of the solid solution, and Ni 3d-O 2p hybridization is not strongly affected, despite the fact that the Ni 2p photoemission changes measurably with increased zinc content. Changes in the Ni 2p XPS satellite structure as zinc is added to the rocksalt lattice are a result of attenuation in nonlocal screening due to decreased adjacent nickel site occupancy.