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 ${\mathrm{Mg}}_{7}\mathrm{Nb}{\mathrm{H}}_{n}$ with n ranging from 16 to 25 by calypso method and first-principles calculations. We readily identified the experimentally observed ${\mathrm{Mg}}_{7}\mathrm{Nb}{\mathrm{H}}_{16}$ hydride and uncovered a stable stoichiometry of ${\mathrm{Mg}}_{7}\mathrm{Nb}{\mathrm{H}}_{19}$ 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 ${\mathrm{Mg}}_{7}\mathrm{Nb}{\mathrm{H}}_{19}$, which forms H-H pairs and has weakened metal-hydrogen chemical bonds compared to the stable ${\mathrm{Mg}}_{7}\mathrm{Nb}{\mathrm{H}}_{16}$ and $\mathrm{Mg}{\mathrm{H}}_{2}$ compounds. Our calculations show that ${\mathrm{Mg}}_{7}\mathrm{Nb}{\mathrm{H}}_{16}$ undergoes a structural phase transition from its $P\overline{4}2m$ phase to a $Fm\overline{3}m$ phase at 75 GPa, and the $Fm\overline{3}m$ 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.