Electronic structure, polaron formation, and functional properties in transition-metal tungstates

Transition-metal tungstates MWO(4) (M = Co, Ni, Cu, Zn) have applications in many areas, including supercapacitors. A good understanding of the electronic structure is essential to understanding their functional properties. Here, we report a first-principles study of the materials using hybrid density-functional calculations. The electronic structure is analyzed with a focus on the nature of the electronic states near the band edges. We find that hole polarons can form at the Co lattice site in CoWO(4) and the O site in NiWO(4), CuWO(4), and ZnWO(4), resulting in the formation of Co(3+) in the former and O(–) in the latter. The electrochemical activity observed in certain tungstate compounds, but not in others, appears to be related to the ability to form hole polarons on the transition-metal ions. The formation energy and migration barrier of the hole polaron in CoWO(4) are also calculated and the results are employed to understand the reported p-type conductivity.