Prediction of Novel High-Pressure Structures of Magnesium Niobium Dihydride

On the basis of a combination of the particle-swarm optimization technique and density functional theory (DFT), we explore the crystal structures of MgH₂, NbH₂, and MgNbH₂ under high pressure. The enthalpy-pressure (H-P) diagrams indicate that the structural transition sequence of MgH₂ is α → γ → δ → ε → ζ and that NbH₂ transforms from the Fm3̅m phase to the Pnma phase at 47.80 GPa. However, MgNbH₂ is unstable when the pressure is too low or too high. Two novel MgNbH₂ structures, the hexagonal P6̅m2 phase and the orthorhombic Cmcm phase, are discovered, which are stable in the pressure ranges of 13.24-128.27 GPa and 128.27-186.77 GPa, respectively. The P6̅m2 phase of MgNbH₂ consists of alternate layers of polymetric NbH₆ and MgH₆ triangular prisms, while the Cmcm phase contains distorted MgH₆ trigonal prisms. The calculated elastic constants and phonon dispersions confirm that both phases are mechanically and dynamically stable. The analyses of density of states (DOS), electron localization function (ELF), and Bader charge demonstrate that a combination of ionic and metallic bonds exist in both P6̅m2 and Cmcm phases. We hope the newly predicted magnesium niobium dihydrides with desirable electronic properties will promote future experimental and theoretical studies on mixed main group-transition metal hydrides.