Reversibility of electrosurface transfer through eutectic interfaces of MeWO₄|WO₃ (Me − Ca, Sr, Ba)

Abstract

It is found that electrosurface transition (EST) through eutectic interfaces of (−/+)WO₃|MeWO₄(+/−) induced by electric field is a reversible process. In the case of the (−/+) polarity, nominally, in the “direct experiment”, macro amounts of WO₃ from a W ⁽⁻⁾₃ brick are drawn in the (+) direction onto the inner surface of MeWO₄ forming a two-phase {ie1070-1} composite. Simultaneously, nonequivalent countertransport of Me ²⁺ within the W ⁽⁻⁾₃ brick occurs, which changes the color of W ⁽⁻⁾₃ from the natural hue to dark-green. Intercalation of Me ²⁺ into W ⁽⁻⁾₃ is proved by several spectroscopic methods. The key role in the EST phenomenon belongs to a nonautonomous electrolytic phase of MeW-s formed on the contact interface with WO₃|MeWO₄. The composition of MeW-s is close to W/Me ≈ 2. As a result of EST, the cell acquires a more complicated structure: {fx1070-1} where |///| are interface regions occupied by the MeW-s phase. At the cathodic boundary of subcell {fx1070-2} the following process occurs: {fx1070-3}, The process at the anodic boundary is: {fx1070-4}. Ultimately, WO₃ is transported in the (+) direction (into the composite) and Me ²⁺ penetrates under the effect of the gradient in chemical potential into W ⁽⁻⁾₃ forming a dark-green Me _x WO₃ phase with its front reaching the (−) Pt electrode. After the end of the “direct experiment”, the cell polarity was changed to (+/−) and the “reverse experiment” was carried out. Now, on the cathodic boundary | ₄ of subcell {fx1070-5} anions (WO₄)²⁻ are generated that are discharged on boundary ₃ | to oxide WO₃ that is intercalated into the right boundary of MeWO_{4 − (3)}, where the rightmost composite region {ie1070-2} is formed. Thus, the mass of W ⁽⁻⁾₃ decreases; it becomes dark-green (see above) and the mass of the MeWO₄ disk continues growing and now its structure is as follows {ie1070-3}. It is important that the left W ⁽⁺⁾₃ disk that was dark-green after the “direct experiment” gradually becomes lighter in the “reverse experiment” up to its natural pale green color, i.e., Me ²⁺ is deintercalated from it: Me ²⁺: Me _x WO₃ + 1/2O₂ → xMe ²⁺ + 2e + WO₃. It is found that dependences of variations of disk masses Δm(Q) practically coincide for the “direct” and “reverse” experiments.