First-principles Study On Structures And Superconductivity Of Novel Metal Hydrides Under High Pressure

Owing to the light atomic mass and high Debye temperature,dense solid hydrogen has been always considered as the most promising high temperture superconductor,but how to obtain metallic hydrogen is a crucial prerequisite.It is well known that the gaseous hydrogen is an insulator at ambient pressure,so it is impossible for hydrogen to exhibit any superconductivity.Under pressures,gas in nature often experiences the transformation from gas to liquid and then to solid state,because distances between atoms shorten in the process of pressurization.More importantly,the band gap usually decreases and the bands occur overlapping under high pressure,which makes the original non-metallic material transition to the metal.Therefore,pressurization is an important method for obtaining solid metal hydrogen.As early as 1935,Huntington and Wigner predicted theoretically that if the pressure is higher than 25 GPa,the molecular hydrogen would be converted to metallic hydrogen.Then in 1968,Ascroft suggested that if the hydrogen could realize metalization,the higher electron density at the Fermi level would help it become a room temperature superconductor.However,experimentally,the metallic hydrogen has not been observed until pressure up to 388 GPa.Although current experiment claims that metallization of solid hydrogen has been observed at 495 GPa,more experiments need to confirm this conclusion.In 2004,Ascroft pointed out that due to the"chemical precompression"from heavy element M,the H in IVA hydrogen-dominated compounds MH₄?M=C,Si,Ge,Sn,Pb?could achieve metallization at lower pressure,and these metallic MH₄ could be candidates for high temperature superconductors.In this paper,the crystal structure prediction technology,namely particle swarm optimization algorithm and genetic algorithm,combined with first principles calculation method based on density functional theory are used to systematically study some binary and ternary hydrides under high pressures.The research content mainly includes high-pressure crystal structures,hydrogen bonding form,metallizations,electronic structures,vibration properties and electron-phonon couplings etc.,the innovative results are as follows:1)The crystal structure and superconductivity of Bi-H compounds under high pressure.A systematic search for stable high-pressure phases of BiH_n?n=1⁶?is performed based on evolutionary algorithm and particle swarm optimization algorithm method.Except for BiH₃,other bismuth hydrides have been predicted to be stable by pressurization.Hydrogen atoms exist in bismuth lattice with three different forms and exhibit primarily ionic feature.Equivalent doping produces Bi H with atomic hydrogen.Excessive impurity renders the emergence of exotic paired H₂ units.In particular,BiH₅ adopts an intriguing layered structure intercalated by1D hydrogen chains composed of H₂ and the linear H₃ units.The predictions of T_c with 29 K for BiH,60 K for BiH₂,83 K for BiH₄,108 K for BiH₅ and 102 K for BiH₆ at high pressures indicate all stable hydrides are potential high-temperature superconductors.Most remarkably,it is importantly uncovered that the T_cs of Bi-H system is dramatically heightened with increasing hydrogen content.In addition,it is H-derived intermediate-frequency vibrations that are mainly responsible for the strong electron-phonon coupling,which is substantially different from solid H₃S.2)Superconductivity of A15 hydride Hf H₃ under high pressure.By using particle swarm optimization algorithm and evolutionary algorithm,we predict an A15 type hydride HfH₃.When the pressure is above 150 GPa,HfH₃ is found to have lower average enthalpy,which is more stable in energy than previously reported HfH₂.When the pressure reaches 300 GPa,we find HfH₂ decomposition,but HfH₃remains stable.In the A15 hydride HfH₃,the electronic structure calculation shows that the electronic state density at the Fermi level is mainly contributed by the d electrons of Hf,meaning that the d electrons of Hf mainly control the conductivity of system.Further analysis indicates that Hf H₃ is an ionic crystal.Using the modified McMillan equation,we find that HfH₃ has a superconducting transition temperature of 20 K at 150 GPa.It is found that HfH₃ has a large total electronic state density in the vicinity of Fermi level?DOS-FL?,but the DOS-FL is not derived by H,which is responsible for a small electron-phonon parameter?=0.69,meaning that a large and H-derived electronic state density at the Fermi level is particularly beneficial for promoting strong electron-phonon coupling.3)High-pressure synthetic path and superconductivity of ternary MgSiH₆.We have predicted a new high pressure ternary hydride,MgSiH₆,which is a metal with ionic character and a simple cubic structure identified with space group Pm-3above 250 GPa.Ab initio calculations show that MgSiH₆ is a potential high-temperature superconductor with a superconducting critical temperature of about 63 K at 250 GPa.The calculations show that there are some soft phonon modes in the phonon spectrum along the?-X and?-M directions.Further analysis shows that phonon softening is mainly caused by the Fermi surface nesting,and it plays a crucial role in the superconductivity of MgSiH₆.In this work,we present a new“triangle straight-line method”,which provides clear guidance for the specific A+B?D synthesis path of the ternary Mg-Si-H hydrides,and it reveals effectively two distinct high pressure synthesis pathways to obtain MgSiH₆,namely MgH₂+SiH₄?MgSiH₆ and MgSi+3H₂?MgSiH₆.This method may be applied to all ternary compounds and it will be very important for further experimental synthesis.4)Synthetic path and superconductivity of ternary hydride MgGeH₆under high pressure.Inspired by the high-temperature superconductivity of some binary hydrogen-rich compounds,we systematically explored the high-pressure phase diagram and superconductivity of the Mg-Ge-H compounds using first-principle method.Through a series of high pressure synthesis routes,the high hydrogen content stoichiometric MgGeH₆ possessing Pm-3 space ground was predicted.We have studied in depth the three different high pressure formation paths of MgGeH₆,namely Mg+Ge+3H₂?MgGeH₆,MgGe+3H₂?MgGeH₆ and MgH₂+GeH₄?MgGeH₆.At 200 GPa,we obtain a ternary MgGeH₆ by directly compressing elemental Mg+Ge+3H₂.By incorporating the MgGe alloy into hydrogen,we have found that MgGeH₆ can form and become stable at 200 GPa.More interestingly,we also predicted the same MgGeH₆ by squeezing MgH₂ and GeH₄ mixtures to 250 GPa.Electron structure calculations reveal the cubic MgGeH₆is a good metal and takes on ionic character.Electronic structure calculations suggest that the cubic MgGeH₆ is a good metal and exhibits an ionic character.At200 GPa,electron-phonon coupling calculation uncovers a large 1.16 of?for MgGeH₆.Particularly,we found that the MgGeH₆ could be a potential high temperature superconductor with superconducting transition temperature of about67 K at 200 GPa.5)Hydrogen-dominating superconductivity in ternary hydrides MAlH₆?M=Li and Mg?under high pressure.We used ab initio methods to explore the stable stoichiometries,crystal structures and properties of M-Al-H?M=Li,Mg?systems via a feasible route MAl+nH?MAlH_n?n=1⁷?under pressure of 150³00GPa.We found that two ternary hydrides LiAlH₆ and MgAlH₆ stabilize above 200GPa and share similar tetragonal structure.Electronic structure calculations reveal that both Li Al H₆ and MgAlH₆ take on primarily ionic character.The most prominent feature is that hydrogen is predominantly responsible for large electronic density of states at the Fermi level?DOS-FL?.In particular,our investigations suggest that it is H-dominated electrons that couple strongly all phonon modes,which results in large electron-phonon coupling parameters?=1.87 and 0.85,and high superconducting transition temperatures T_c of¹60 and 77 K for LiAlH₆ and MgAlH₆ at 200 GPa,respectively.We also found that the improvement of H-derived DOS-FL is a very promising strategy to enhance superconductivity of hydrides,which gives a strong motivation to design or explore some novel high temperature superconductors in other ternary hydrides.