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 MgH 2, NbH 2, and MgNbH 2 under high pressure. The enthalpy–pressure ( H – P ) diagrams indicate that the structural transition sequence of MgH 2 is α → γ → δ → ε → ζ and that NbH 2 transforms from the Fm 3̅ m phase to the Pnma phase at 47.80 GPa. However, MgNbH 2 is unstable when the pressure is too low or too high. Two novel MgNbH 2 structures, the hexagonal P 6̅ m 2 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 P 6̅ m 2 phase of MgNbH 2 consists of alternate layers of polymetric NbH 6 and MgH 6 triangular prisms, while the Cmcm phase contains distorted MgH 6 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 P 6̅ m 2 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.