Na$^+$-ion conductivity of double phosphate Na$_3$Sc$_2$(PO$_4$)$_3$ in the region of the $\beta$–$\gamma$ transition

The Na$^+$-ion conductivity $\sigma$ of double phosphate Na$_3$Sc$_2$(PO$_4$)$_3$ in the region of the $\beta$–$\gamma$ transition has been studied using impedance spectroscopy (1–10$^6$ Hz). The polycrystalline sample of Na$_3$Sc$_2$(PO$_4$)$_3$ has been prepared by solid-phase synthesis and ceramic technology. It has been found that, upon the $\beta$–$\gamma$ transition, the conductivity $\sigma$ of Na$_3$Sc$_2$(PO$_4$)$_3$ suffers a $\sim$ 1.5-fold jump at 470 $\pm$ 2 K upon heating and a $\sim$ 2.5-fold jump at 430 $\pm$ 4 K upon cooling (the temperature hysteresis of the jump in $\sigma$ is 40 K). For double sodium-scandium phosphate $\gamma$-Na$_3$Sc$_2$(PO$_4$)$_3$ in the superionic state, $\sigma$ attains 0.07 S/cm at 700 K and the ion transport activation enthalpy is 0.42 $\pm$ 0.02 eV.