Strong anisotropy in the electromagnetic properties of Na$_{2}$Ti$_{2}X_{2}$O ($X =$ As, Sb) crystals

Na$_{2}$Ti$_{2}X_{2}$O ($X =$ As, Sb) crystals have been grown from the flux method. X-ray diffraction characterization revealed an anti-K$_{2}$NiF$_{4}$-type layered structure (tetragonal, space group $I$4\textit{/mmm}) for both compounds. Magnetic susceptibility ($\chi (T )$) and electrical resistivity ($\rho$($T$) measurements revealed major kinks at 115 K ($T_{s1})$ and 320 K ($T_{s2})$ for Na$_{2}$Ti$_{2}$Sb$_{2}$O and Na$_{2}$Ti$_{2}$As$_{2}$O, respectively, signifying possibly the opening of density wave gaps. Both Na$_{2}$Ti$_{2}$Sb$_{2}$O and Na$_{2}$Ti$_{2}$As$_{2}$O showed remarkably strong anisotropy in their electromagnetic transport properties, and values of $\gamma_{\rho } (\rho_{c}/\rho_{ab})$ even reached 140 and 430, respectively, being much larger than that of iron pnictide BaFe$_{2}$As$_{2}(\gamma_{\rho} =$ 2--5). The $\gamma \rho $of Na$_{2}$Ti$_{2}$Sb$_{2}$O changed slightly with cooling, though a small drop at $T_{s1}$ occurred. In contrast, the $\gamma_{\rho }$ of Na$_{2}$Ti$_{2}$As$_{2}$O changed strikingly by exhibiting not only a small change at $T_{s2}$ but also a sudden decrease of 50 K, reduced nearly 1$/$3. Specific heat measurement indicated that Na$_{2}$Ti$_{2}$Sb$_{2}$O was only partially gapped with $\gamma_{1} =$ 4.1 mJ mol$^{-1}$ K$^{-2}$, though a long-range order was established at $T_{s1}$, while Na$_{2}$Ti$_{2}$As$_{2}$O was fully gapped. The remarkably strong electromagnetic anisotropy revealed in Na$_{2}$Ti$_{2}X_{2}$O suggests the crucial role of the TiO$_{2}X_{4}$ layer for the transport properties of layered titanium oxypnictides.