Hydrogen absorption and its effect on low-temperature electric properties of niobium

The excess resistivity of niobium foil samples due to doped hydrogen increases with concentration up to 8–12 at.% H, which is almost independent of temperature. The Nb-H structure at room temperature or above is quenched to 77 K or 9.5 K by relatively rapid cooling. The abrupt decrease of resistivity is observed, which corresponds to the precipitation of the ordered orthorhombic hydride phase. The superconducting transition temperature Tc is rather insensitive to hydrogen concentration in α solid solution. Hydride phase, however, shows no sign of superconductivity down to 1.3 K. Hydrogen is found to be reabsorbed in niobium after UHV outgassing by storage in air or by immersion in etchants. This is confirmed by weighing the samples and cross-checked by the proton NMR measurements.