Prediction of Novel High-Pressure Structures of Magnesium Niobium Dihydride

Abstract

On the basis of a combination of the particle-swarm optimization technique and density functional theory (DFT), we explore the crystal structures of MgH, NbH, and MgNbH under high pressure. The enthalpy-pressure (H-P) diagrams indicate that the structural transition sequence of MgH is α → γ → δ → ε → ζ and that NbH transforms from the Fm3̅m phase to the Pnma phase at 47.80 GPa. However, MgNbH is unstable when the pressure is too low or too high. Two novel MgNbH 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 MgNbH consists of alternate layers of polymetric NbH and MgH triangular prisms, while the Cmcm phase contains distorted MgH 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.

Pressure-Driven Structural Phase Transitions and Superconductivity of Ternary Hydride MgVH6

“…for MgVH 6 are considered as also shown in Figure 1, where the corresponding stable phases P4 2 /mnm, Pnma, and P6 3 /mmc of MgH 2 4,6 and the stable phases Fm3̅ m and Pnma of VH 2 53 are employed at the corresponding pressure. At the same time, the P6 3 /mmc and Im3̅ m phases for magnesium, 54−56 Im3̅ m and Fm3̅ m phases for vanadium, 57,58 and P6 3 / m and C2/c phases for hydrogen 59 are adopted in all calculations.…”

“…Usually, α-MgH 2 adopts a TiO 2 -rutile-type crystal structure at atmospheric pressure, 4 and presents some novel phases γ (Pbcn) and β (Pa3̅ ) under high pressure. 5,6 Similar as other hydrogen-rich binary compounds, 7,8 various unconventional stoichiometric Mg−H binary systems have been synthesized at high pressure, and most of them have high superconductivity. It was reported that the superconducting critical temperature (T c ) of the sodalite-type MgH 6 can reach up to 260 K at 300 GPa.…”

“…MgH 2 , as a promising solid-state hydrogen storage material, has attracted considerable attention among the class of metal hydrides due to its high hydrogen storage capacity (7.6 wt% H 2 ), the availability of raw materials, and lower costs. − The great potential of MgH 2 in hydrogen storage has stimulated extensively research on its crystal structure and superconductivity. Usually, α-MgH 2 adopts a TiO 2 -rutile-type crystal structure at atmospheric pressure, and presents some novel phases γ ( Pbcn ) and β ( Pa 3̅) under high pressure. , Similar as other hydrogen-rich binary compounds, , various unconventional stoichiometric Mg–H binary systems have been synthesized at high pressure, and most of them have high superconductivity. It was reported that the superconducting critical temperature ( T c ) of the sodalite-type MgH 6 can reach up to 260 K at 300 GPa .…”

Phase diagrams and electronic properties of B-S and H-B-S systems under high pressure

“…The recent progress on this theme can be found in review articles [21][22][23][24][25][26][27]. However, ternary hydrides might also be potential high-temperature superconductors [28][29][30][31][32][33], and some studies have shown that they not only show high superconducting transition temperatures (T c ) but also exhibit unexpected superconducting mechanisms. For example, the predicted ternary hydride MgSiH 6 has a T c value of 63 K at 250 GPa, and its superconducting transition mainly comes from phonon softenings along → X and → M directions [34].…”

Possible Martensitic Transformation in In2(Mo,W)X (X = Cr, Mn, Fe, Co, and Ni) Heusler Alloys

“…First-principles calculations have been widely used to study the physical properties of various compounds including the Heusler alloys, perovskite oxide, yttrium orthoaluminate, rhodium silicides, and carbon dioxide [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. To probe the cubic structural ground state and martensitic transition of In 2 (Mo,W)X (X = Cr, Mn, Fe, Co, and Ni) alloys, we carry out first-principles density-functional calculations with CASTEP code where the plane-wave basis set is implemented [27,28].…”