Ionic conductivity of double sodium–scandium and cesium–zirconium molybdates

The electrical properties of the Na$_3$Sc(MoO$_4$)$_3$ and Cs$_2$Zr(MoO$_4$)$_3$ compounds are investigated using impedance spectroscopy (1–10$^6$ Hz) in the temperature range 100–650$^\circ$C. Double molybdates in the form of a fine-crystalline powder are obtained by solid-phase synthesis in air at 450–600$^\circ$C for 20–50 h. It is found that the temperature dependence of the ionic conductivity of ceramic samples exhibits anomalies at temperatures of 605 $\pm$ 5$^\circ$C for Na$_3$Sc(MoO$_4$)$_3$ and 425 $\pm$ 15$^\circ$C for Cs$_2$Zr(MoO$_4$)$_3$ due to the phase transitions, which are confirmed by the data of thermal analysis. Above the superionic transitions, the ionic conductivity reaches 0.084 S/cm (650$^\circ$C) for Na$_3$Sc(MoO$_4$)$_3$ and 0.002 S/cm (462$^\circ$C) for Cs$_2$Zr(MoO$_4$)$_3$.