  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.
