Superionic transitions and conductivity anisotropy in Na$_{4.6}$FeP$_2$O$_{8.6}$F$_{0.4}$ single crystals

The cation conductivity in the directions parallel $(\sigma_{\parallel[001]})$ and perpendicular $(\sigma_{\perp[001]})$ to the [001] crystallographic direction of Na$_{4.6}$FeP$_2$O$_{8.6}$F$_{0.4}$ single crystals has been investigated at 293–734 K. The specific features of the ionic conductivity have been studied near two phase transitions at $T_{\mathrm{tr},1}\sim$ 450 K and $T_{\mathrm{tr},2}\sim$ 545 K. At $T = T_{\mathrm{tr},1}$, the activation enthalpy for the dependences $\sigma_{\parallel[001]}(T)$ and $\sigma_{\perp[001]}(T)$ decreases from 0.45 $\pm$ 0.01 to 0.33 $\pm$ 0.02 eV, and the $\sigma_{\parallel[001]}(T)$ curve has a jump of the ionic conductivity by a factor of almost two at $T = T_{\mathrm{tr},2}$; the jump is related to a manifestation of commensurate modulation of the crystal structure. In the Na$_{4.6}$FeP$_2$O$_{8.6}$F$_{0.4}$ crystals, the ionic transport is anisotropic with the ratio $\sigma_{\parallel[001]}/\sigma_{\perp[001]}$ = 7.7, 5.2, and 6.6 at 293 K $(T <T_{\mathrm{tr},1})$, 500 K $(T_{\mathrm{tr},1}<T<T_{\mathrm{tr},2})$, and 700 K $(T <T_{\mathrm{tr},2})$, respectively. The mechanism of cation conductivity in the Na$_{4.6}$FeP$_2$O$_{8.6}$F$_{0.4}$ crystals is discussed.