The nature and the mechanism of ion transfer in tungstates Me²⁺{WO₄} (Ca, Sr, Ba) and Me ³⁺₂ {WO₄}₃ (Al, Sc, In) according to the data acquired by the tubandt method

Abstract

The Tubandt method of electrolysis is used for studying the nature of ionic carriers in ceramics of tungstates Me²⁺{WO₄} (Ca, Sr, Ba) and Me ³⁺₂ {WO₄}₃ (Al, Sc, In) which are solid electrolytes. These compounds have the salt-like islet structure with isolated tetrahedrons {WO ²⁻₄ } and are crystallized in the allied structural types of scheelite (CaWO₄) for Me²⁺ and Sc tungstate (Sc₂{WO₄}₃) for Me³⁺. The electrolysis is carried out in 2- or 3-section cells (−)Pt|M ^n/n+₂ {WO₄}|Me ^n/n+₂ {WO₄}|Pt(+) in air atmosphere at the temperature of ∼900°C and cell voltage of 4 and 300 V. All experiments without exception demonstrate a decrease in the mass of the cathodic section of cells. This points to the negative charge of ionic mass carriers and their transfer towards the Pt⁽⁺⁾ electrode. The cathodic briquette mass loss Δm ⁽⁻⁾ depends linearly on the charge passed through a cell. In all experiments with MeWO₄ tungstates, the anodic disk mass remains constant. The electrolysis of Me₂(WO₄)₃ cells is always accompanied by an increase in the anolyte mass Δm ⁽⁺⁾; however, in all experiments, Δm ⁽⁻⁾ > Δm ⁽⁺⁾. All data on mass variation and the results of studying the composition of nearelectrode electrolyte layers by XRD and SEM methods correspond to the condition $t_{WO_4^{2 - } } > t_C$ (C is the cation), i.e., {WO ²⁻₄ } anions pertain to the major ionic carriers. The transport number $t_{WO_4^{2 - } }$ is calculated based on the Faraday law from Δm ⁽⁻⁾. It is shown that the second ionic carrier with the mobility even higher than that of {WO ²⁻₄ } is the O²⁻ ion. For middle values of transport numbers, their ratio is shown to be $t_{O^{2 - } }$(0.5–0.8) > (0.2–0.5)$t_{WO_4^{2 - } }$. No results that would confirm the involvement of Me²⁺ and Me³⁺ ions in conduction are obtained.