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

Abstract

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 HxAWO4 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.