Structure of Polaronic Centers in Proton-Intercalated A WO 4 Scheelite-Type Tungstates.

The studies of polaronic centers in a homologous series of scheelite-type compounds A WO 4 ( A = Ca, Sr, Ba) were performed using the W L 3 -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 H x A WO 4 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 6+ to W 5+ , i.e., the formation of polaronic centers, and giving rise to a characteristic dark blue-purple color. The emergence of the W 5+ was confirmed by XPS experiments. The relaxation of the local atomic structure around the W 5+ 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 5+ ions from the center of [W 5+ O 4 ] tetrahedra in the structure of A WO 4 scheelite-type tungstates. This finding was also supported by the results of the first-principles calculations.