First-principles Study On Crystal Structures And Superconductivity Of Molybdenum Hydrides Under High Pressure

Numerous hydrides are high temperature superconductors under high pressure.Since synthesizing of materials and measurement of physical properties under high pressure is hard to conduct,the crystal structure prediction method based on first-principles calculation is a powerful tool to study the high pressure phases of hydrides.In reality,many hydrides with high superconducting critical temperatures were predicted theoretically before confirmed experimentally.Though superconductivity of most binary hydrides under high pressure has been investigated,superconductivity of molybdenum hydrides under high pressure has not been studied yet.Using first-principles calculation and the crystal structure prediction method based on particle swarm optimization,we investigated crystal structures and physical properties of molybdenum-hydrogen compounds under high pressure systematically.The main results are as follows:1.Crystal structure searches for Mo₄H_4n(n=1-6)at 150 GPa and 300 GPa found three new stable phases,namely P6₃/mmc-MoH,Cmcm-MoH₅ and Pnma-MoH₆ as well as three new metastable phases with low enthalpies,namely C2-MoH₄,I4/m-MoH₄ and P2₁/m-MoH₆.2.Zero-point motion affects thermodynamic stability of molybdenum-hydrogen compounds under high pressure.Under 300 GPa,MoH,MoH₂,MoH₃,MoH₄ and MoH₅ are stable phases without zero-point energy while MoH₃ and MoH₄ are no longer stable with zero-point energy.3.The experimentally confirmed P6₃/mmc-MoH transforms into the newly predicted P6₃/mmc-MoH under 133 GPa.The primitive unit cell of the former phase contains4 atoms while that of the latter phase contains 8 atoms.4.Cmcm-MoH₅ and P2₁/m-MoH₆ are good superconductors with critical temperatures of 77 K and 67 K at 300 GPa,respectively,which is due to their large electron-phonon coupling of 1.34 and 1.13,respectively.Our work will promote understanding of hydrides under high pressure and be helpful in future experiment.