Structure of Polaronic Centers in Proton-Intercalated WO Scheelite-Type Tungstates.

The studies of polaronic centers in a homologous series of scheelite-type compounds WO ( = Ca, Sr, Ba) were performed using the W L-edge and Sr K-edge X-ray absorption spectroscopy combined with the reverse Monte Carlo simulations, X-ray photoelectron spectroscopy (XPS), and first-principles calculations. Protonated scheelites HWO were produced using acid electrolytes in a one-step route at ambient conditions. The underlying mechanism behind this phenomenon can be ascribed to the intercalation of H into the crystal structure of tungstate, effectively resulting in the reduction of W to W, i.e., the formation of polaronic centers, and giving rise to a characteristic dark blue-purple color. The emergence of the W was confirmed by XPS experiments. The relaxation of the local atomic structure around the W polaronic center was determined from the analysis of the extended X-ray absorption fine structures using the reverse Monte Carlo method. The results obtained suggest the displacement of the W ions from the center of [WO] tetrahedra in the structure of WO scheelite-type tungstates. This finding was also supported by the results of the first-principles calculations.