First-principles Study On The Structural Stability And Superconducting Properties Of Metal Hydrides

According to Bardeen-Cooper-Schrieffer?BCS?theory,the superconducting critical temperature?T_c?is proportional to the Debye temperature,and the superconducting transition temperature is proportional to the Debye temperature.Metal hydrogen is expected to become a high-temperature superconductor due to its extremely high Debye temperature and strong electro-phonon coupling?EPC?.However,after a lot of experiments,the pressure was applied to 388 GPa,and no metallization of H was found,and hydrogen is still an insulator.After first-principles calculations,it was found that H can form a metal state at 450 GPa and exhibit superconductivity,with a superconducting transition temperature of 242 K.Recently,it has been reported that solid H will produce metallicity at 495 GPa,and this conclusion is very controversial.There is no direct evidence that the conclusion is correct,and more experiments are needed to confirm this conclusion.Since metal hydrogen cannot be obtained under general experimental pressure conditions,one way to reduce the pressure is to study metal hydrides.Ashcroft showed in 2004 that relatively heavier elements in metal hydrides will exert a strong"pre-compression"effect on H,which can realize the metallization of hydrides at a lower pressure.Guide premise.Extensive theoretical studies have explored the potential superconductivity in compressed hydrogen-rich materials.According to reports,the crystal structure search technology combined with the first-principles calculation of Im-3m H₃S at 200 GPa T_c is 203 K,this theoretical prediction was proved by subsequent experimental work.In H₃S,H element plays a leading role in superconducting properties,and S also plays a role in stabilizing the configuration of H atom.The surprising discovery of H₃S's high-temperature superconductivity broke the previous record of 164 K in copper-based superconductors and took a solid step in finding new hydride high-temperature superconductors.The high temperature superconductivity of H₃S inspired us to study the superconducting properties of hydrogen-rich compounds.In addition to containing H elements,non-H elements are also very important for the entire system.Encouraged by H₃S high-temperature superconductors,people began to pay attention to the research of metal hydrides.In previous theoretical calculation studies,first-principles calculations predicted the potential high-temperature superconductivity of rare earth hydrides.For example,Y-H compounds are predicted to be high-temperature superconductors.At 400 GPa,the superconducting transition temperature of Fm-3m YH₁₀ can reach 303 K.In hydrides of lanthanides,the superconducting transition temperature of La H₁₀ at 150 GPa reaches 215 K,and compounds with higher superconducting transition temperatures may appear in lanthanide metal hydrides.In this paper,the first-principles calculation method is used to theoretically predict the crystal structure,pressure stable phase diagram,electronic properties,phonon properties and superconductivity of some binary and ternary hydrides under high pressure,and obtain some Theoretical research results:In the Ce-H compound theoretically predicted under high pressure,the superconducting transition temperature of F-43m Ce H₉ at 94 GPa is about 142 K,and the superconducting transition temperature of Fm-3m Ce H₁₀ at 94 GPa is 168 K.This indicates that the rare earth metal may have a low-frequency vibration associated with heavy atoms due to its large atomic mass,resulting in a large electro-phonon coupling ? The element Sm has extremely strong metallicity,and the hydride generated by combining with H atoms may be metallic,which is beneficial to high-temperature superconductivity,and the hydride of Sm may be metallized under a lower pressure.In the next work,we use crystal structure prediction software combined with first-principles calculation methods to conduct in-depth studies on the structural stability,electronic properties,dynamic properties and superconducting properties of Sm H_n?n=2-10?compounds.Within the set pressure range,Sm H₂,Sm H₃,Sm H₄,Sm H₇,Sm H₉,Sm H₁₀ have stable structures,among which Sm H₄,Sm H₉ and Sm H₁₀ which may become high temperature superconductors at 56 GPa,600 GPa and600 GPa maximum superconducting transition temperature Reached 114 K,243 K and 293 K respectively.Among them,I4/mmm Sm H₄ may become a superconductor at a lower pressure,which is attributed to its higher contribution of H atoms to the density of electronic states at the Fermi surface,showing metallicity,and having a higher coupling strength of electrophonons.After that,we studied the ternary metal hydride system.The maximum superconducting transition temperature of P6₃/mmc Ce HSe at 100 GPa is 9.2 K.At this time,the coupling constant of the electrophonon,the density of electronic states at the Fermi surface,and the phonon frequency The logarithmic averages were 0.875,0.7659,and 166.0356.The maximum superconducting transition temperature of R-3m Ce Mg₂H₇ at 20 GPa is 16.3 K.At this time,the R-3m Ce Mg₂H₇ electrophonon coupling constant,the density of electronic states at the Fermi surface,and the average logarithm of the phonon frequency are 0.9896,6.1328,and 239.2738 respectively.When P4₁2₁2 Sm Mg₂H₇ is at0 GPa,the maximum superconducting transition temperature is 0.095 K,which is close to 0.Therefore,we believe that Sm Mg₂H₇ has no superconductivity.This is because its electro-phonon coupling parameter ? is too small,resulting in a very low T_c value.The ternary hydride due to its more complex structure,and the possible hybridization of the doping element makes the low frequency region of the system contribute the most to the total electrophonon coupling strength,which may result in a lower ? value,which makes the whole system The superconducting transition temperature is relatively low.Although the superconducting transition temperatures of these ternary metal hydrides are not very high,they can provide some reference suggestions for the research of ternary hydrides under high pressure,and provide important guidance for the exploration of other ternary metal hydride high-temperature superconductors.