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=m* 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 m≈3.0×10E−18 [m**2* K]