  The chemical states of Ni, Zn, and W in microcrystalline NiWO$_4$-ZnWO$_4$
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
L$_2$M$_{23}$M$_{45}$, Zn L$_3$M$_{45}$M$_{45}$, and W N$_4$N$_{67}$N$_{7}$
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 NiWO$_4$ 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 [NiO$_6$] and
[WO$_6$] 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).
