Effect of oxygen concentration on superconducting properties of rubidium tungsten bronzes Rb _x WO _y

Superconducting transition temperature (T _c) as a function of oxygen concentration for hexagonal rubidium tungsten bronzes Rb _x WO _y with 2.80 ≤ y ≤ 3.07 and x = 0.19, 0.23, and 0.27 has been systematically investigated. Three regions corresponding to T _c < 2 K (denoted as superconductivity suppressed region), T _c∼ 3 K (superconductivity uniform region) and T _c > 3 K (superconductivity enhanced region) were identified in T _c-y phase diagram for Rb _0.19WO _y and Rb _0.23WO _y. No superconductivity enhanced region was observed for Rb _0.27WO _y. The superconductivity suppressed region shifts toward higher oxygen content as rubidium concentration increases. The local ordering of the intercalated rubidium atoms might be responsible for the intriguing T _c-y phase diagram of Rb _x WO _y.