Fig. 1: X-ray diffraction (XRD) patterns collected at 38 and 117 GPa at different temperatures.

Temperatures are indicated. Nb, NaCl, and MgO Bragg peaks are labeled. In order to highlight the detection of the recrystallization and melting phenomena at 38 GPa, we have excluded XRD patterns where the orthorhombic Pnma structure is detected to coexist with body-centered cubic bcc-Nb; examples of such bcc-Pnma coexistence are shown in Fig. 3.