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Although niobium is a well characterized material, it still shows some anomalies that are not yet understood. Therefore, we revisit its metastable phases using density functional theory. First, we systematically compare energies and ground-state volumes of chosen crystal structures and discuss possible transition paths to the bcc ground-state structure and the energy landscape for tetragonal distortions. Furthermore, we discuss their stability by means of their phonon spectra and vibronic free energies. Second we analyze the impact of tantalum impurities on phase stability. We find additional aspects of the energy landscape of the material: A local energy minimum on the bcc to ω transition path, a flat energy landscape with respect to uniaxial strain along [111], and a considerable stabilization of the σ phase by tantalum substitution.