The dipole ordering and the ionic conductivity in the NASICON-like structures of the Na$_{3}$Sc$_{2}$(PO$_4)_{3}$ type

Problems of the dipole ordering and ionic conductivity of Na$_{3}$Sc$_{2}$(PO$_4)_{3}$ polycrystal that has three polymorphic phases – $\alpha$, $\beta$, and $\gamma$ – are studied. The features of the $\alpha$-Na$_{3}$Sc$_{2}$(PO$_4)_{3}$ crystal structure and the dipole ordering and relaxation polarization in the $\alpha$ and $\beta$ phases are specified. The origin of the dipole ordering in the $\alpha$ phase and the partial dipole disordering in the $\beta$-Na$_{3}$Sc$_{2}$(PO$_4)_{3}$, as well as the high ionic conductivity in the $\beta$ and $\gamma$ phases of the Na$_{3}$Sc$_{2}$(PO$_4)_{3}$ polycrystal are associated with phase transformations $\alpha\to\beta$, $\beta\to\gamma$ that result in structural changes of the rhombohedral $\{[\mathrm{Sc}_2(\mathrm{PO}_4)_3]^{3-}\}_{3\infty}$ crystalline framework. A model explaining the emergence of the dipole ordering and the ionic conductivity phenomena in Na$_{3}$Sc$_{2}$(PO$_4)$ is proposed.