Electrical conductivity of a single crystal of polar dielectric NaNO$_2$ under temperature exposure in a vacuum

Impedance spectroscopy was used to study the temperature dependence of the direct-current electrical conductivity $\sigma_{dc}(T)$ of a NaNO$_2$ single crystal (space group Im2m) oriented along the [010] polar direction under vacuum heating conditions of 10$^{-1}$ Pa. Sodium nitrite crystals were obtained from a melt using the Kyropoulos technique. Impedance measurements were performed in three heating runs, 293–470 K. It was found that vacuum heating of the NaNO$_2$ crystal results in a change from an ionic to a semiconductor mechanism of electrical conductivity. Upon initial heating, the ionic conductivity of the crystal is equal to 7 $\times$ 10$^{-7}$ S/cm at 470 K. During the second and third heating runs, the conductometric data coincide, and the temperature dependence $\sigma_{dc}(T)$ exhibits a semiconductor character with an activation energy of 81 $\pm$ 2 meV for electrical transfer. The conductivity $\sigma_{dc}$ increases by a factor of 70, reaching 5$\times$10$^{-5}$ S/cm (470 K). The mechanisms of ionic and semiconductor conductivity in the NaNO$_2$ crystal are discussed.