The nature and the mechanism of ion transfer in tungstates Me2+{WO4} (Ca, Sr, Ba) and Me3+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 2 3+ {WO4}3 (Al, Sc, In) which are solid electrolytes. These compounds have the salt-like islet structure with isolated tetrahedrons {WO 4 2− } 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 2 /+ {WO4}|Me 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 4 2− } 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 4 2− } 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.