  Niobium is one of the most researched superconductors, both theoretically and
experimentally. It is enormously significant in all branches of superconducting
applications, from powerful magnets to quantum computing. It is, therefore,
imperative to understand its fundamental properties in great detail. Here we
use the results of recent microscopic calculations of anisotropic electronic,
phonon, and superconducting properties, and apply thermodynamic criterion for
the type of superconductivity, more accurate and straightforward than a
conventional Ginzburg-Landau parameter $\kappa$ - based delineation, to show
that pure niobium is a type-I superconductor in the clean limit. However,
disorder (impurities, defects, strain, stress) pushes it to become a type-II
superconductor.
