The presence of small quantities of oxygen and nitrogen in interstitial positions in the vanadium lattice is found to affect markedly the superconductive properties of the metal. X-ray evidence supports the supposition that these impurities set up internal strains which are known to give rise to properties very similar to those of the hard superconductors. It is suggested that these strains, which, unlike those arising from mechanical work, are not always removable by vacuum heat treatment, are responsible for the difficulties associated with the preparation of samples of the metals exhibiting a reversible $B$, $H$ curve.

It is shown that the sharp penetration fields for a relatively pure specimen are probably not very different from the equilibrium fields. The transition temperature of vanadium is 5.13°K, ${\left(\frac{d{H}_c}{\mathrm{dT}}\right)}_{T_c}=436\mathrm{}\pm 20$ oersteds/deg, and the Sommerfeld $\gamma =15\mathrm{}\times {10}^{-4\mathrm{}}$ cal/mole deg ${\mathrm{K}}^2$.

• Received 21 September 1951

DOI:https://doi.org/10.1103/PhysRev.85.85