Experimental and theoretical confirmation of an orthorhombic phase transition in niobium at high pressure and temperature

Daniel Errandonea,John E Proctor,Leonid Burakovsky,David Santamaría-Perez,Malcolm I Mcmahon,Mohamed Mezouar,Simon G Macleod,Dean L Preston,Shaoping Chen,Hyunchae Cynn,Sergey I Simak

doi:10.1038/s43246-020-00058-2

Abstract

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.

Highlights

Compared to other body-centered cubic transition metals, Nb has been the subject of fewer compression studies and there are still aspects of its phase diagram which are unclear
Of special interest is HP-HT polymorphism in the bcc transition metals of Group 4B (Cr, Mo, and W) and Group 5B (V, Nb, and Ta), which is related to laser-heated diamond anvil cell (DAC) melting experiments bringing up a small slope of the corresponding melting curves in the pressure–temperature (P–T) coordinates in the megabar P range[11,12,13,14,15,16,17,19]
X-ray diffraction (XRD) measurements reported here are from seven runs carried out on fresh samples to avoid the influence of possible chemical reactions on the results

Summary

Introduction

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. Of special interest is HP-HT polymorphism in the bcc transition metals of Group 4B (Cr, Mo, and W) and Group 5B (V, Nb, and Ta), which is related to laser-heated diamond anvil cell (DAC) melting experiments bringing up a small slope (dT/dP) of the corresponding melting curves in the pressure–temperature (P–T) coordinates in the megabar P range[11,12,13,14,15,16,17,19] These flat melting curves contradict the results of both more recent experiments[18,20,21,22,23,24,25] and calculations[2,4,6,8,9,10]. In order to advance our knowledge of the phase diagrams of transition metals, we report ab initio calculations on the Nb phase diagram to 450 GPa and X-ray diffraction (XRD) experiments on Nb to 120 GPa and 6000 K

Methods

Results

Conclusion