Compared to other body-centered cubic (bcc) transition metals, Nb has been the subject of fewer compression studies and there are still aspects of its phase diagram which are unclear. Here, we report a combined theoretical and experimental study of Nb under high pressure and temperature. We present the results of static laser-heated diamond anvil cell experiments up to 120 GPa using synchrotron-based fast x-ray diffraction combined with ab initio quantum molecular dynamics simulations. The melting curve of Nb is determined and evidence for a solid-solid phase transformation in Nb with increasing temperature is found. The high-temperature phase of Nb is orthorhombic Pnma. The bcc-Pnma transition is clearly seen in the experimental data on the Nb principal Hugoniot. The bcc-Pnma coexistence observed in our experiments is explained. Agreement between the measured and calculated melting curves is very good except at 40–60 GPa where three experimental points lie below the theoretical melting curve by 250 K (or 7%); a possible explanation is given.

与其他以人体为中心的立方（bcc）过渡金属相比，Nb的压缩研究较少，而且其相图的某些方面尚不清楚。在这里，我们报告了在高压和高温下对Nb进行理论和实验相结合的研究。我们介绍了基于同步加速器的快速X射线衍射与从头算起的量子分子动力学模拟相结合，静态激光加热的金刚石砧座细胞实验的结果，最高可达120 GPa。确定了Nb的熔化曲线，并发现了Nb随温度升高而发生固-固相变的证据。Nb的高温相为斜方晶系Pnma。在Nb主要Hugoniot的实验数据中可以清楚地看到bcc-Pnma转变。解释了在我们的实验中观察到的bcc-Pnma共存。测量和计算的熔解曲线之间的一致性非常好，只是在40–60 GPa时，三个实验点比理论熔解曲线低250 K（或7％）。给出了可能的解释。