Superconductivity of Vanadium

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.