Superconductivity of sodium tungsten bronze with cubic structure

Sodium tungsten bronze NaxWO3 with cubic structure is reported to exhibit bulk superconductivity at sodium content 0.16 less than or equal to x less than or equal to 0.4. The superconducting transition temperature T-c and the temperature dependence of the critical field H-c2 have been measured for the samples with different x. It was concluded that the enhancement of T-c at low x-values occurs due to softening of the phonon spectrum. The possible reason for positive curvature of the H-c2 vs T curve is discussed.*