Trap identification and impurity-induced localization of muons in Nb

We present new muon-spin-rotation ($\mu \mathrm{SR}$) results on extremely purified and very well characterized niobium samples with defined minute amounts of Ta or N. This allows us to separate the influence of substitutional and interstitial impurities on the $\mu \mathrm{SR}$ spectra. The purest sample shows a low but nonzero $\mu \mathrm{SR}$ linewidth from 0.1 to 70 K, while Ta doping gives rise to a high linewidth below 20 K. Pure N doping creates the broad maximum between 30 and 70 K, but also a considerable linewidth below 20 K. The results are interpreted in terms of a model where initial localization is partly catalyzed by impurities and where some muons do not form small-polaron states.