Dipole ordering and ionic conductivity in NASICON-Type Na$_{3}$Cr$_{2}$(PO$_{4}$)$_{3}$ structures

The aspects of structure, dipole ordering, and ionic conductivity of the Na$_{3}$Cr$_{2}$(PO$_{4}$)$_{3}$ crystal with the four polymorphic phases ($\alpha$, $\alpha'$, $\beta$, and $\gamma$) have been investigated. The features of the $\alpha$-Na$_{3}$Cr$_{2}$(PO$_{4}$)$_{3}$ crystal structure and its dipole ordering and relaxation polarization in the low-temperature $\alpha$, and $\alpha'$ phases have been refined. The occurrence of Na$_{3}$Cr$_{2}$(PO$_{4}$)$_{3}$ dipole ordering in the $\alpha$ and $\alpha'$ phases and high ionic conductivity in the $\beta$- and $\gamma$ phases is attributed to the structural changes in the rhombohedral $\{[Me_{2}$(PO$_{4}$)$_{3}$]$^{3-}\}_{3\infty}$ crystal frame upon phase transformations $\alpha\to\alpha'$, $\alpha'\to\beta$, and $\beta\to\gamma$. A model for explaining the dipole ordering and ionic conductivity phenomena in Na$_{3}$Cr$_{2}$(PO$_{4}$)$_{3}$ is proposed.