Chemical-state analyses of Ni, Zn, and W ions in NiWO4–ZnWO4 solid solutions by X-ray photoelectron spectroscopy

Abstract

The chemical states of Ni, Zn, and W in microcrystalline NiWO4–ZnWO4 solid solutions were studied by X-ray photoelectron spectroscopy. The recorded spectra of the Ni 2p, Zn 2p, and W 4f photoelectron lines and Ni L2M23M45, Zn L3M45M45, and W N4N67N7 Auger-transition lines show pronounced changes with increasing Zn concentration. The positions of the resolved photoelectron and Auger-transition lines were combined to construct so-called chemical-state plots (Wagner or Auger-parameter plots) for metal ions in solid solutions. With increasing Zn concentration, the Auger parameter increases for Ni and decreases for W, thus evidencing a lowering and an increase of the electronic polarizability around core-ionized Ni and W ions, respectively. At the same time, the character of Zn–O bonds and the local structure around Zn ions do not change. It is concluded that the dilution of NiWO4 with Zn ions is accompanied by an increase of the Ni–O bond ionicity and an increase of the W–O bond covalency. These changes are attributed to the charge redistribution among [NiO6] and [WO6] structural units. We show that a careful in-depth analysis of XPS data obtained with a laboratory-based X-ray photoelectron spectroscopy system can give chemically sensitive, qualitative information on the changes in the first coordination spheres of each metal ion. This information is otherwise accessible only by synchrotron-based techniques (such as X-ray absorption spectroscopy).