Theoretical Studies On Structures And Superconductivity Of Several Metal Superhydrides Under High Pressure

Pressure is one of the thermodynamic parameters that could control the properties of condensed materials.The pressure could directly change the distance of atoms in the substance.With high pressure,the bond length is shortened,the band gap is reduced,the energy band is extended and occur overlapping,and then metallization is realized in the original non-metallic material.Therefore,pressurization is considered as an effective and important method for obtaining solid metal hydrogen which is probably a high temperature superconductor.as we all know,at ambient conditions,the hydrogen is an insulator.Owing to the light atomic mass and high Debye temperature(48),based on superconducting BCS theory,the superconducting transition temperature T_c is proportional to the Debye temperature(48),therefore the scientists have always considered the dense solid hydrogen as one of promising high temperature superconductors,even room temperature superconductors.But obtaining metallic hydrogen is an essential and crucial question.In the high-pressure experiments,the metallic hydrogen was not observed until today.Although there are experimental scientists claimed that they have observed the metallization of solid hydrogen,but the experimental results are still controversial and require further confirmation."Chemical precompression"of non-hydrogen element was proposed by Ashcroft at 2004.In hydrogen-rich compound,the pressure of metallization of hydrogen would reduce,and the metallic hydride could exhibit high superconducting transition temperature T_c.In this paper,we mainly use the first principles calculation method,combined with the crystal structure prediction technology to systematically study lanthanum hydrides La H₁₀,some heavy rare earth hydrides and ternary Na-Al-H,Ba-Re-H hydrides under high pressures.Here,in the thesis,the high-pressure crystal structures,hydrogen bonding form,electronic structures,dynamical stability,electron-phonon couplings(EPC),and superconductivities etc were mainly studied.For the lanthanum hydrides La H₁₀,we obtained consistent isotope coefficient and superconducting transition temperature with experimental results,based on that,we further investigated the superconducting mechanism of La H₁₀.In the heavy rare earth hydrides,we uncover three such sodalite-type clathrate hydrides,stable Tm H₆,Yb H₆ and metastable Lu H₆(T_c=145 K at P=70 GPa for Yb H₆ and,especially,T_c=273 K at P=100 GPa for Lu H₆).These striking properties are a consequence of the strong interrelationship between the f states present at the Fermi surface,stability of structure and T_c value.For the ternary Na-Al-H system,most of hydrides contain H₂units and H^-.More importantly,we found Na Al H₈ exhibits superconducting transition temperature T_c of 55 K at 300 GPa.For the ternary Ba-Re-H system,we found two stable ternary hydrides:Cmmm-Ba₂Re H₈ and Pm3(?)-Ba Re H₁₂.Ba₂Re H₈ possesses novel anion[Re H₈]^4-.Pm3(?)-Ba Re H₁₂ is stable at 100 GPa,H-H distance is about 1(?),and the prediction of superconducting transition temperature is 128 K.The specific several innovative results are as follows:(1)High T_c superconductivity in Heavy Rare Earth hydrides XH₆(X=Tm,Yb,Lu)The novel superhydrides at high pressure show great potential for high temperature superconductivity,but these new hydrogen-based superconducting materials mostly exist above 150 GPa,and such high pressure greatly limits the application and research of these superconducting materials.Therefore,obtaining high temperature superconducting phase under lower pressure is currently a key scientific problem in this field.In addition,rare earth metal hydrides have novel structures and physical images,and it is urgent to summarize the general laws affecting superconductivity.Based on this,we have systematically investigated the structural stability and superconductivity properties of heavy rare earth element(Tm,Yb,Lu)hydrides at high pressures.We uncover three such sodalite-type clathrate hydrides,stable Tm H₆,Yb H₆and metastable Lu H₆.The heavy rare earth element XH₆(X=Tm,Yb,Lu)cage hydride has the potential to exhibit high superconducting transition temperature.T_c=145 K at P=70 GPa for Yb H₆ and,especially,T_c=273 K at P=100 GPa for Lu H₆,reaching the freezing point of ice.For XH₆,the striking properties are a consequence of the strong interrelationship between the f states present at the Fermi surface,structural stability,and T_cvalue.For example,Tm H₆,with unfilled 4f orbitals and the Fermi surface is dominated by f electrons,is stable at 50 GPa,while it has a relatively low value of T_c of 25 K.The Yb H₆ and Lu H₆ compounds,with their filled f-shells,and as a result,an increase in T_c.The phonon spectrum and the Eliashberg spectral function reveals that the f-electrons at the Fermi surface favor the stability of the cubic XH₆ cage hydride structure,but not its superconductivity.(2)High T_c state and isotope coefficient of lanthanum hydrides:La H₁₀.The high₍(8) phase of superhydride La H₁₀ deserves special attention because this phase displays the high critical temperature 250-260K at high pressure,which is another breakthrough after the high-temperature superconductivity of H₃S.We focus on the evaluation of the critical temperature,isotope coefficient and their pressure dependence of this phase(Fm3(?)m phase of the La H₁₀ compound).By combining the method of two coupling constants,?_opt and?_ac,and two characteristic frequencies with first-principle calculations we are able to analyze the role of the light hydrogen in the amplitude of the critical temperature in this unique compound.Specifically,the critical temperature of 254 K and the isotope coefficient of 0.45 are evaluated at high pressure,this is a good agreement between the experimental observation and theoretical calculated results.With the increase of pressure,the isotope coefficient remains basically the same,the electroacoustic coupling strength weakens,and the superconducting transition temperature decreases accordingly.By analyzing the electronic structure,phonon spectrum and elastic constants,it is found that the existence of electronic pockets in the Fermi surface leads to the peak of the electronic pockets in the Fermi surface leads to the peak of the electronic density of states near the Fermi surface,the elastic constant C₄₄ causes phonon softening,and the optical branch and the vibration of hydrogen play a major role in the electroacoustic coupling,revealing that the high superconducting transition temperature of La H₁₀ is mainly attributed to the higher electronic density of states and stronger EPC controlled by hydrogens.(3)Phase diagrams and superconductivity of ternary Na-Al-H compounds at high pressure Recently,many breakthroughs have been made in the superconductivity study of ternary hydride at high pressure:theoretically predicted Li₂Mg H₁₆ with superconducting transition temperature of more than 400 K at 400 GPa;C-S-H compounds with room temperature superconductivity(288 K)at 267 GPa were found in high pressure experiments.The study about the superconductivity of ternary hydride under high pressure is becoming popular.We systematically studied the phase diagram,electronic properties,dynamic stability and superconductivity of ternary Na-Al-H system under high pressure by the first principle method.We found stable Na Al H₄,Na Al H₆ and Na Al H₈,metastable Na Al H₇.According to the electronic structure,Na Al H₇ and Na Al H₈ with H-and H₂ units are metallic,and Na Al H₄ and Na Al H₆ are good semiconductor.In the metallic Na-Al-H ternary compound,Na Al H₈has a T_c of 55 K at 300 GPa.There is a phonon softening phenomenon caused by Fermi plane nesting along the high symmetry point A-M in phonon spectrum.Phonon softening enhances the electron-phonon coupling and is beneficial to its superconductivity.In addition,we propose four feasible synthesis paths:Na+Al+H₂,Na+Al H₃+H₂,Na H+Al+H₂,and Na H+Al H₃+H₂ to provide reference for the subsequent high-pressure experimental synthesis of ternary Na-Al-H compounds.Our work provides important guidance for the further design of poly-hydrides with high superconducting transition temperature.(4)Phase diagrams and superconductivity of ternary Ba-Re-H compounds at high pressure Based on the Ba-H,Re-H system and existing Ba Re H₉ at ambient conditions,we systematically explored phase diagrams,electronic properties,lattice dynamics and superconductivity of ternary Ba-Re-H system under high pressures using first-principle methods.In the pressure range of 100-300 GPa,stable structures Pm3(?)-Ba Re H₁₂ and Cmmm-Ba₂Re H₈ were found.In Cmmm-Ba₂Re H₈,we found[Re H₈]^4-anion ligands.In Ba₂Re H₈,the electronic density contribution of H at the Fermi level is small,which is not conduce to the coupling of hydrogen electrons with medium and high frequency phonons,and the T_cis only 19 K at 100 GPa.As the pressure increases,the electron-phonon coupling strength of Ba₂Re H₈ becomes weak and the superconducting transition temperature T_c decreases.In Pm3(?)-Ba Re H₁₂ which is stable at 100GPa,H also exists in the form of H₂ units,and the H-H spacing is about 1(?).Its energy band structure has both flat and steep energy bands,and the proportion of H at the Fermi level is relatively high.At 100 GPa,the theoretical prediction of superconducting transition temperature T_c is 128K.The H in the metastable P2/m-Ba Re H₁₂ exists in the form of H₂ units,and the H-H distances are within the range of 0.88?1.07(?),that means they are weak covalent bond;its superconducting transition temperature T_cgradually increases with the increase of pressure,and the T_c is 41.6 K at300 GPa.Our work may provide important guidance for further searching for multi-hydride which is high-temperature superconductors.