Influence of composition nonstoichiometry on the electrical conductivity of LiNaGe$_4$O$_9$ crystals

The electrical conductivity $\sigma$ of Li$_{2-x}$Na$_x$Ge$_4$O$_9$ ($x$ = 1, 0.5, 0.2) crystals in an alternating-current electric field has been investigated at a frequency of 1 kHz in the temperature range of 300–800 K. A considerable anisotropy of the electrical conductivity has been revealed for crystals with a sodium concentration $x$ = 1 at $T >$ 500 K. It has been shown that the electrical conductivity $\sigma$ along certain crystallographic directions increases by more than three orders of magnitude with a change in the sodium concentration from $x$ = 1 to $x$ = 0.2. The results have been discussed taking into account the specific features of the structure of the crystals under investigation. Presumably, the major charge carriers are interstitial Li ions migrating along channels of the framework structure of the Li$_{2-x}$Na$_x$Ge$_4$O$_9$ crystals.