Conduction properties of the hexagonal tungsten bronze,RbxWO3

Studies of the electrical transport properties—resistivity, Hall effect, and Seebeck coefficient—of the hexagonal tungsten bronze ${\mathrm{Rb}}_x\mathrm{W}{\mathrm{O}}_3$ for $0.16<~x<~0.33$ are reported for the temperature range from 1.5 to 300 K. In the normal state, these properties show temperature-dependent anomalies which suggest a phase transition with temperature. The temperature at which this transition occurs is shown to depend on the Rb concentration, exhibiting a sharp maximum near $x=0.25\mathrm{}$. As the Rb concentration is decreased below $x=0.33\mathrm{}$ the superconducting transition temperature first increases above the value of 1.94 K at $x=0.33\mathrm{}$ and then decreases to less than 1 K at $x=0.24\mathrm{}$. As the Rb concentration is lowered further a second increase in $T_c$ is observed. This behavior appears to be associated with a phase transition at a composition near $x=0.25\mathrm{}$. The nature of neither the transition with temperature nor the transition with concentration could be precisely determined. The superconducting state shows a large temperature-independent 60° anisotropy in $H_{c2\mathrm{}}$ in the plane perpendicular to the hexagonal axis and positive curvature in the temperature dependence of $H_{c2\mathrm{}}$ near $T_c$.