Figure 1

The crystal structure of $\beta -A_X{\text{V}}_2{\text{O}}_5$. The ${\text{V}}_2{\text{O}}_5$ framework consists of three crystallographically independent vanadium atoms (V1, V2, and V3) and oxygen atoms, which are illustrated as chromatically different ${\text{VO}}_5$ pyramids and ${\text{VO}}_6$ octahedra. This framework has tunnels along the $b$ axis, and $A$ cations are aligned in this tunnel. At the stoichiometric composition $x=1/3$, the ratio of ${\text{V}}^{4+}/\text{V}$ ions is $1/6$ in the case of $A^{+}$ cations and is $1/3$ in $A^{2+}$ cations. The stoichiometric composition $x=0.33$ $(1/3)$ is a singularity point where the system shows metallic behaviors. The slight nonstoichiometry (even if $x=0.32$ or 0.34) of $A$ cations spoils its quasi-1D metallic property. This means that the 1D electronic state on the ${\text{V}}_2{\text{O}}_5$ framework is very sensitive to imperfection in the Na ion array in the tunnels along the $b$ axis, probably since the Anderson localization is not negligible in a 1D system.Reuse & Permissions

Figure 2

Temperature dependence of resistivity under high pressures. (a) An overall range view. (b) A macrograph of the low-temperature region below 50 K. The inset in the lower panel exhibits a current-voltage curve at 4.2 K, 8.0 GPa.Reuse & Permissions

Figure 3

Temperature dependence of ac susceptibility under high pressures. Drops in ac susceptibility under several pressures show the shielding effect of a superconducting transition. The insets show a shielding signal of Pb metal as a reference and a schematic view of the measurement method.Reuse & Permissions

Figure 4

The pressure-temperature $(P-T)$ phase diagram of $\beta -{\text{Na}}_{0.33}{\text{V}}_2{\text{O}}_5$. $T_N$ (filled triangle) AF transition temperature, $T_{\mathrm{C}\mathrm{O}}$ (filled square) charge-ordering temperature, $T_{\mathrm{S}\mathrm{C}}$ (circle) superconducting transition temperature. The filled and blank symbols of $T_{\mathrm{S}\mathrm{C}}$ represent the onset temperature in resistivity and ac susceptibility measurements, respectively.Reuse & Permissions