The nature and the mechanism of ion transfer in tungstates Me ~(2+){WO_4} (Ca, Sr, Ba) and Me _2 ~(3+) {WO_4}~3 (Al, Sc, In) according to the data acquired by the tubandt method

摘要

The Tubandt method of electrolysis is used for studying the nature of ionic carriers in ceramics of tungstates Me~(2+){WO_4} (Ca, Sr, Ba) and Me _2 ~(3+) {WO_4}~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 (Sc_2{WO_4}_3) for Me~(3+). The electrolysis is carried out in 2- or 3-section cells (-)Pt|M _2 ~(n+) {WO_4}|Me _2 ~(n+) {WO_4}|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 MeWO_4 tungstates, the anodic disk mass remains constant. The electrolysis of Me_2(WO_4)_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_(WO4) ~(2-) > t _C (C is the cation), i.e.; {WO _4 ~(2-) } anions pertain to the major ionic carriers. The transport number $t-{WO-{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 O~(2-) 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_(WO4) ~(2-). No results that would confirm the involvement of Me~(2+) and Me ~(3+) ions in conduction are obtained.