Dipole ordering and the ionic conductivity in Na$_{3}$Fe$_{2}$(PO$_{4}$)$_{3}$-type NASICON-like structures

The problems of the structural features, the dielectric and conducting properties of Na$_{3}$Fe$_{2}$(PO$_{4}$)$_{3}$ polycrystals prepared by hot pressing have been studied. The Na$_{3}$Fe$_{2}$(PO$_{4}$)$_{3}$ polycrystals in the $\alpha$ and $\beta$ phases are shown to have higher conducting properties than the $\alpha$ and $\beta$ phases of the single crystals grown by the solution–melt method. The regularities of the appearance of the dipole ordering, the ionic and superionic conductivities related to phase transitions and structural changes in the $\{$[Fe$_{2}$(PO$_{4})_{3}]^{3-}\}_{3\infty}$ rhombohedral crystalline frame during polymorphic transformations are refined. The problems of the relaxation polarization in the $\alpha$ and $\beta$ phases of Na$_{3}$Fe$_{2}$(PO$_{4}$)$_{3}$ are discussed. A model for explaining the dipole ordering and the ionic conductivity in Na$_{3}$Fe$_{2}$(PO$_{4}$)$_{3}$ is proposed.