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 P4̄2m phase to a Fm3̄m phase at 75 GPa, and the Fm3̄m 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.