First-principles Study Of Several New Hydrides Under High Pressure

Since the concept of superconductivity was first proposed by Onner et al.in 1911,the search for a new superconductor with a high critical temperature has become one of the important topics in the science of condensed matter.Based on the microscopic superconducting theory BCS theory,the superconducting transition temperature Tc is proportional to the Debey temperature of the system,while the Debey temperature is inversely proportional to the atomic mass of the system.Therefore,metal hydrogen is considered to be the most ideal material for realizing a room temperature superconductor,but solid hydrogen is an insulator at ambient pressure.Pressure is one of the thermodynamic parameters that control the structure and properties of condensed matter.The high pressure can effectively squeeze the distance inside the atom of the substance,shorten the bond length,extend the energy band,reduce the band gap,and realize metallization of the insolated systems.What's more,the pressure can reduce the energy barriers of some chemical reactions,and then form new compounds that are difficult to synthesize at atmospheric pressure.Therefore,high pressure is considered to be an effective method for achieving metallization of the insulating hydrogen solid,and futhur realizing its room temperature superconductivity.However,in high pressure experments,hydrogen failed to become metallic even under 388 GPa.Although one experiment have suggested the discovery of solid metal hydrogen at pressures up to 495 GPa,the experimental results are controversial and still require further experimental confirmation.In 2014,Ashcroft speculated that if we doped other elements into pure hydrogen,the "chemical pre-compression" effect introduced by impurity atoms would reduce the pressure of metallization,and thus achieve room temperature hydrogen-rich superconductors under high pressures.In this paper,we mainly use the crystal structure prediction technology combined with the first-principle calculation method to systematically study the hydrogen-rich compounds of the binary At-H system and Ca-H system and the ternary LiP-H system under high pressure.Here,the crystal structures,bonding mothods,the types of hydrogen atoms in the hydrides,electronic structures and superconductivities of several novel hydrides are deeply analyzed.What's more,we first discussed the effects of spin-orbit coupling effect on hydrogen-rich superconducting compounds under high pressure in the At-H system;in the Ca-H system,the new stable phase under high pressure is discovered and two new phases with high Tc values are revealed;in LiP-H system,the addition of lithium can stabilize the metastable P_H system.We found the stable lithium phosphorus terary hydrdids and calculated their superconductivities are found under high pressure.The innovative results are as follows:(1)Stable structure and superconducting properties of At-H system with spin-orbit effect under high pressure.In the study of hydrogen-rich compound systems,although the contribution of hydrogen is the main factor affecting the superconductivity of the system,the influence of non-hydrogen elements on the system is still not negligible.Therefore,in the process of exploring hydrogen-rich compounds,it is necessary to understand the influence of non-hydrogen elements on the system.We used first-principles calculation method to study the crystal structures,electronic structures and superconductivities of the At-H system in the pressure range of 50-300 GPa.The results show that AtH2 is stable at 50 GPa,phase transition Cmcm Pnma occurs at 70 GPa;AtH4 is stable at 60 GPa,and phase transition P6/mmm Cmmm occurs at 220 GPa.The electronic structures indicate that AtH2 and AtH4 are metallic in the whole pressure range of the study and the electronic structures of At-H system are mainly dominated by At atoms.It is predicted that both AtH2 and AtH4 are superconductors,and the Tc is about 5-10 K and 30-50 K in the pressure range of 50-300 GPa,respectively.Further analysis shows that in P6/mmm-AtH4,the first acoustic branch dominated by vibration of astatine has a pressure-induced phonon softening phenomenon,which is closely related to the phase transition of P6/mmm Cmmm in AtH4 and resulting in an increase in the electron-phonon coupling(EPC)parameter ?in P6/mmm-AtH4.More importantly,our work shows that the spin-orbit coupling(SOC)effect of At element has great influences on the ground state energies,the phase transition pressure points,the electronic structures,and even the superconductivities of the At-H system.This effect may also occur in other hydride systems containing heavy elements,and the results we have obtained may provide some suggestions for the study of other heavy atom hydrides.(2)Unique Phase Diagram and Superconductivity of Calcium Hydrides.Recent theoretical works have shown that rare earth metal hydrides can form high Tc hydrides YH10 and LaH10 containing H atom cages with Tc of 303 K and 286 K at 400 GPa and 210 GPa,respectively.Later,LaH10 was synthesized in a high pressure experiment and the Tc was measured to be higher than 250 K.The discovering of the high Tc in rare earth hydrides made people pay more attention to alkaline earth metal hydrides who has some similarities in high pressure phase diagram and superconductivities to rare earth metal hydrides.In addition,in the past experimental and theoretical studies,the phase diagram of the Ca-H system above 200 GPa has not been explored,so we use the first-principles calculation method to investigate the Ca-H system in the pressure range of 50-400 GPa.The phase diagram was systematically studied and the results of previous work were reproduced.What's more,we found that in the Ca-H system the new ratio of calcium hydrogen compound CaH9 is stable in the pressure range of 100-400 GPa,and phase transition occurs at 167 GPa and 235 GPa,respectively.The phase transition sequence is Cm P21/m C2/m.According to the current research results,CaH9 may be the only alkaline earth metal hydride with an odd hydrogen content.Due to the diagonal relationship between Ca and Y,the stability of the Ca-H system under high pressure is similar to that of the Y-H system.However,since the Ca atom cannot provide sufficient electrons to H,the H atoms of the high-pressure phase C2/m-CaH9 does not form the same H29 covalent cages as those in P63/mmc-YH9 but forms H2 and H3 molecules.Furthermore,at 400 GPa we predicted a metastable phase R-3m-CaH10 which is isostructural with the stable phase R-3m-SrH10 in the Sr-H system,in which the hydrogen atoms exist in a pleated honeycomb two-dimensional layered structure.It is worth noting that the new phase C2/m-CaH9 and the metastable phase R-3m-CaH10 are potential high Tc superconductors with Tc values of approximately 266 K and 175 K at 300 GPa and 400 GPa,respectively.Electronic structure analysis shows that good superconductivities are likely because the electronic density values N(?f)near the Fermi level are dominated by H elements.Our research results may provide some references and suggestions for further experiments and theoretical work on the calcium hydrogen system under high pressure.(3)The stable structures and superconductivities of ternary compounds LiPHn(n=1-7)under high pressure.In the experiment of pressurized PH3,Eremets et al found that the Tc of 103 K was measured when the pressure reached 207 GPa,but the structural information of this superconductive phases was not given.Subsequent theoretical work found that there is no phosphorous compound that is stable relative to P and H2 in the pressure range above 100 GPa.The experimental work found that PH3 will decompose into elemental P and H2 above 35 GPa.In the periodic table of elements,phosphorus is adjacent to sulfur,the predicted phosphorus hydrides have the similar bonding method to sulfur hydrides.Therefore,we can stabilize the metastable phosphorous hydrides by introducing an additional electron.We added lithium to the P-H system and hope that lithium can stabilize metastable phosphorous hydrides under pressure.We searched the stable structures of ternary lithium phosphorus hydrides under high pressure and studied their superconductivities.We have found that lithium can stabilize the metastable P-H system under high pressure and propose a possible synthetic route for lithium-phosphorus ternary hydride:LiP+H2 ? LiPHn.We discovered new lithium-phosphorus hydride ternary hydrides LiPH3,LiPH4,LiPH6 and LiPH7 which are stable under high pressure.Except for the atomic phase of Pm-3-LiPH6,P-H bonds exist in each stable structure.The electronic properties indicate that the low pressure phases C2/c-LiPH6 and P-1-LiPH7 are non-metallic phases,and the remaining stable structures are all metallic.In the metallic phase,the Tc values of P2/m-LiPH3 and C2/m-LiPH7 are about 49.7-67.8 K and 46.8-60.5 K at 200 and 300 GPa,respectively.It is worth noting that the Tc of phase Pm-3-LiPH6 is as high as 180.4 K at 200 GPa and reduces as pressure increasing.The atomic phase Pm-3-LiPH6 has the same hydrogen content as the metastable compound PH3.Significantly,Pm-3-LiPH6 has a higher Tc value under pressure which is closely related to the light masses of Li atoms and large contribution of the vibrations of hydrogen atoms to the electron-phonon coupling.Our work may provide some advice for further researches on ternary hydrides.