Structure of Polaronic Centers in Proton-Intercalated AWO₄ Scheelite-Type Tungstates

The studies of polaronic centers in a homologous series of scheelite-type compounds AWO₄ (A = Ca, Sr, Ba) were performed using the W L₃-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 HWO₄ 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 W⁶⁺ to W⁵⁺, i.e., the formation of polaronic centers, and giving rise to a characteristic dark blue-purple color. The emergence of the W⁵⁺ was confirmed by XPS experiments. The relaxation of the local atomic structure around the W⁵⁺ 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 W⁵⁺ ions from the center of [W⁵⁺O₄] tetrahedra in the structure of AWO₄ scheelite-type tungstates. This finding was also supported by the results of the first-principles calculations.