Ternary Mg-Nb-H polyhydrides under high pressure

Magnesium based alloys are promising solid materials for hydrogen storage. However, it is a real challenge to synthesise hydrogen storage materials with high hydrogen storage capacity and low dehydrogenation temperature. Here, we have performed extensively structural searches for ternary magnesium-based hydrogen storage compounds of Mg7NbHn with n ranging from 16 to 25 by CALYPSO method and first-principles calculations. We readily identified the experimentally observed Mg7NbH16 hydride and uncovered a stable stoichiometry of Mg7NbH19 with high hydrogen storage capacity of 6.7 wt% and low dehydrogenation temperature of 273 K. The remarkable decrease of the hydrogen release temperature is attributed to the atomic rearrangements in Mg7NbH19, which forms H-H pairs and has weakened metal-hydrogen chemical bonds compared to the stable Mg7NbH16 and MgH2 compounds. Our calculations show that Mg7NbH16 undergoes a structural phase transition from its P4 over bar 2m phase to a Fm3 over bar m phase at 75 GPa, and the Fm3 over bar m phase is a potential polyhydride superconductor. The present findings offer insights for understanding the hydrogen storage and release of Mg-Nb-H ternary magnesium-based hydrogen storage compounds, which open avenues for the design and synthesis of magnesium-based hydrogen storage material.