The addition of interstitial solute atoms to niobium, in concentrations below the solubility limit, lowers the transition temperature. Interstitial oxygen has the largest effect, decreasing $T_c$ by 0.93°K per at.%; while increasing the resistivity in the normal state by 5.2 μΩ cm per at.%. Magnetization curves obtained on niobium ($\frac{R_{298°\mathrm{K}}}{R_{10°\mathrm{K}}}\approx 500$) and on similar specimens containing interstitial oxygen or nitrogen are substantially reversible and are similar to the shape predicted by Abrikosov for superconductors of the second kind. The field first penetrating the sample, $H_{\mathrm{FP}}$, less than the thermodynamic critical field, $H_c$, decreases with increasing concentration of the interstitial atom, while $H_{\mathrm{N}}$, the field at which the normal state is restored (as determined from magnetization measurements), increases. The ratio, $\frac{H_{\mathrm{N}}}{H_c}$, is a linear function of ${\rho }_n$. When the solubility limit is exceeded, hysteresis effects become more pronounced similar to those predicted by Bean's model. Resistive measurements at low current density in longitudinal magnetic fields indicate that both niobium and its interstitial solid solutions exhibit some superconducting properties above $H_{\mathrm{N}}$.

• Received 25 April 1963

DOI:https://doi.org/10.1103/PhysRev.132.107