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 $\omega$ transition path, a flat energy landscape with respect to uniaxial strain along [111], and a considerable stabilization of the $\sigma$ phase by tantalum substitution.

中文翻译：

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Nb 和 Ta 取代 Nb 的（亚）稳定相之间的转变路径的从头研究

尽管铌是一种特性良好的材料，但它仍然表现出一些尚未了解的异常现象。因此，我们使用密度泛函理论重新审视其亚稳态相。首先，我们系统地比较了所选晶体结构的能量和基态体积，并讨论了 bcc 基态结构的可能跃迁路径和四方扭曲的能量景观。此外，我们通过声子谱和振动自由能讨论了它们的稳定性。其次我们分析了钽杂质对相稳定性的影响。我们发现了材料能量景观的其他方面：bcc 上的局部能量最小值$\omega$过渡路径，相对于沿[111]的单轴应变的平坦能量景观，以及相当大的稳定性$\sigma$通过钽替代相。