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In order to study the nature of phase transitions along the magnetization curve of low-κ superconductors, several Ta-, Nb- and V- single crystals of varying impurity content and, hence, different Ginzburg-Landau parameter κ (Tc) were investigated. The results are compared with recent microscopic calculations valid at arbitrary impurity content, but restricted to the weak-coupling isotropic limit. For the type-I/type-II/1 phase boundary, the differences between theory and experiment are attributed to anisotropy effects. On the type-II side of the phase diagram, theory predicts a significant dependence of the phase boundary on the impurity concentration, whereas the experiments fall on a rather uniform boundary curve.

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