Correlation between transition temperature and crystal structure of niobium, vanadium, tantalum and mercury superconductors

The bond length (x) of transition metals Nb, V, Ta and Hg represents the shortest atomic separation in a crystal unit cell. It is suggested that there exists a strong correlation between (x) and the inverse of the critical superconducting transition temperature Tc in the form (2x)2 Neff=m2 1/Tc. Here Neff is the number of electrons in the outermost s-shell. The slope of the fitted straight line has a value of m2≈3.0×10−18 m2 K.