Theoretical Study Of Metallic Solid Hydrogen And Transition Metal Hydrides Under High Pressure

Hydrogen is the lightest and simplest element on the periodic table.At ambient pressure and temperature,it is a diatomic gas with the molecular formula H2.While it will undergo phase transitions and possess many exotic properties upon compression.Pressure can reduce interatomic distances and profoundly modifies electronic orbitals and bonding patterns.More than 80 years ago,Wigner and Huntington predicted that solid molecular hydrogen would dissociate at high pressure to form a metallic atomic solid.In 1964,Ashcroft proposed that if hydrogen could become metallic,the energy of its ionic vibrations would be so high that even a moderately strong electron–ion coupling could result in high superconducting transition temperature?Tc?.Then plenty of theoretical and experimental works have been performed on the structure,metallic,and superconductivity of hydrogen under high pressure.Unfortunately,lowtemperature studies up to 388 GPa have not yet realized metallization of solid hydrogen.Recently,the metallization of solid hydrogen has been reported in experiment at a ultrahigh pressure of 495 GPa but additional experimental measurements are still needed to testify this claim.The apparent experimental evidence is the observed enhancement of the reflection.While,the XRD diffraction pattern and Raman spectroscopy are difficult to meansure under such extreme pressure and relevant data are lacking.Therefore,the metallization pressure of solid hydrogen and the corresponding phase space structure require further investigations.Here,we have systematically investigated the phase diagram of solid hydrogen in the pressure range of 300-600 GPa by utilizing the density functional theory.Through the literature search,we obtained seven different candidates of solid hydrogen with symmetry of C2/c-12,Pbcn,C2/c-24,Cmca-12,Cmca-4,Fddd and I41/amd.Then the structures are relaxed with GGA-PBE and two other semi-local exchange–correlation?vdW-DF1 and vdW-DF2?functionals.The zero-point vibration energy has also been included.After the comprehensive consideration of thermodynamical and dynamical stability,we obtained candidate structures and corresponding stable pressure ranges.According to GGA-PBE and vdW-DF1,the metallization of high-pressure solid hydrogen occurs blow 300 GPa,contradicting with the pressure range suggested by experiments.vdW-DF2 phase diagram predict that metallic solid hydrogen occurs through the molecular–molecular phase transition at about 485 GPa,and the atomic phase will occur above 600 GPa.We think,the relative stability of phases predicted by vdW-DF2 are reasonable,and the metallization pressure point is accordance with the recent experimental results.With current high-pressure experimental techniques,the metallization of hydrogen is difficult to achieve.The idea on metallic superconducting hydrogen has been expanded into hydrogen-rich compounds owing to the“chemical precompression”,and these compounds can metallize at much lower pressures than the upper limit of current high-pressure techniques.Recently,the novel sulfur hydrides H3S with Im-3m symmetry was predicted theoretically to be a high-temperature superconductor with value of²00 K and then was confirmed in experiment.Recently,the transition metal hydrides,YH6,LaH10 and YH10 have been predicted to possess remarkably high Tc values above 250 K.In this work,the phase diagram,crystal structures,electronic properties,and superconductivity of some transition metal hydrides?Ta-H,V-H,Ti-H?are deeply investigated.In Ta-H system,the species of Ta5H,Ta2H and TaH2 have been synthesized and investigated at ambient pressure.The other stoichiometries and the potential superconductivity of Ta-H compounds are worthy of study.We have searched for the crystal structures of Ta-H system up to 300 GPa by utilizing the evolutionary structure searches.TaH and TaH2 are found thermodynamically stable in the explored pressure range.The other stoichiometries become thermodynamically stable upon compression beyond 50 GPa for TaH3 and TaH4,270³00 GPa for TaH6,respectively.Electronic properties of Pnma?TaH2?,R-3m?TaH4?,and Fdd2?TaH6?show that they are all metallic phases with strong ionic feature,which is different from many hydrogen rich compounds.Further electron–phonon coupling calculations indicate that all of Pnma?TaH2?,R-3m?TaH4?,and Fdd2?TaH6?are superconductors with the estimated Tc of 5.4K-7.1 K,23.9-31 K and 124.2-135.8 K,respectively.Inspired by the high Tc?above 100 K?in TaH6,we then performed systematically simulations on phase diagram,crystal structures and electronic properties of vanadium hydrides under high pressure.Four species,namely VH,VH2,VH3,VH5 are found to be stable under high pressure.The structures and phase transition in VH2 are consistent with the previous study.Based on DFT total energy calculations,we have discovered several new V-H compounds that have not been found before.Three stable stoichiometries of VH,VH3,and VH5 are found to shown strong ionic feature as a result of charge transfer from V to H.The analysis of projected electronic density of states?PDOS?and three-dimensional Fermi surfaces indicate that they are all metallic.The calculations of electron-phonon coupling reveal the potential superconductive vanadium hydrides with estimated superconducting critical temperature?T_c?values of 6.5-10.7 K for R-3m?VH?,8.0-1.6 K for Fm-3m?VH₃?,and 30.6-22.2 K for P6/mmm?VH₅?with the pressure ranging from 150 GPa to 250 GPa.With increasing pressure,the decreasing tendency of?and Tc in P6/mmm?VH5?are mainly attributed to the descending N??F?and increasing phonon frequencies.Our findings are helpful in understanding the structures and superconductivity of the V–H system.As for Ti-H compounds,the structure and phase diagram of TiH2 have been investigated theoretically and experimentally.While the other stoichiometries of Ti-H compounds are barely investigated under high pressure.In this work,the ground-state phases and properties of Ti-H system under high pressures have been extensively explored by utilizing an evolutional local random structural prediction method combined with first principle calculations.The species of TiH and TiH2 are found to be stable over the explored pressure range,and the other stoichiometries become thermodynamically stable upon compression,beyond 25 GPa for TiH3 and175 GPa for TiH6.The stable stoichiometries remain essentially unchanged even the zero-point vibrations is considered.Moreover,the structures and phase transition in TiH2 are in good agreement with the previous experimental and theoretical works.The electronic properties of TiH,TiH3,and TiH6 show that they are all metallic at200 GPa.Further calculations of electron-phonon coupling reveal the potential superconductive titanium hydrides with estimated superconducting critical temperature?Tc?values of 11.8 K for TiH,3.5K for TiH3,and 79.3 K for TiH6 at 200GPa.The negative pressure dependency of Tc and?in TiH6 are mainly attributed to the descending N??F?and the increasing phonon frequencies upon compression.The predicted stable phases are in the range of current high-pressure techniques,and we hope our predictions will stimulate further experimental attempt on the synthesis and conductivity measurements of these titanium hydrides.