Study On Superconductivity And Hydrogen Storage Capacities Of Several Novel Hydrides Under High Pressure

Hydrogen is likely to be a good candidate for the room-temperature superconductor due to its light atomic mass and high Debye temperature.However,hydrogen is a typical molecular crystal under normal pressure,exhibiting insulating characteristic.As one of the thermodynamic parameters that control the structure and properties of materials,pressure can shorten the distance between atoms.As a result of the rearrangement of the crystal lattice,the phase transition occurs and the band gap decreases.Therefore,pressure is an effective means to change the molecular hydrogen in the insulating state into metallic atomic hydrogen.However,the metallization of hydrogen has never been ccachieved",let alone the measurement of its superconductivity.On the contrary,the researches on hydrides have been developing rapidly,in which the metallization and high Te can be achieved under relatively low pressure owing to the"chemical precompression".It seems that the realization of room temperature superconductor is no longer a mirage.Moreover,with the exhaustion of conventional energy exploitation,coupled with the urgent environmental problems to be solved,the development of environment-friendly clean energy is urgent.Hydrogen,with high energy density,good combustion performance,sustainable and safe product of combustion,is recognized as the most promising clean energy in the 21st century.In order to make clean and efficient hydrogen energy available for large-scale utilization,the research on hydrogen storage materials is the top priority.Here,we mainly studied the crystal structures,the types of the hydrogen atoms and superconductivity of the Hf-H,Zr-H and Y-Ca-H systems.The hydrogen storage capacities of K-Si-H structures were also computed.With the first-principle calculation method,a new structure model for superconducting hydrides with high Tc was predicted:"penta-graphene like" HfHlo.Combining first-principles calculations with high-pressure experimental techniques,we successfully synthesized new zirconium hydrides ZrH3 and Zr4H15,then calculated and measured their superconductivity.A "clathrate?like" high-temperature superconductor YCaH12 was predicted to be stable.And a potential hydrogen storage material K2SiH6 was found.The detailed results are as follows:(1)A "penta-graphene like,high temperature superconductor HfHio was designed.A host of hydrogen-rich compounds exhibit good superconductivity under high pressure.Among them,the hydrides with high Te are mainly of two types,one is covalent metallic superconductors represented by H3S,the other is clathrate-like alkaline earth metal and rare earth metal hydride(R)EH6,9,10 represented by LaHlo.The hydrogen atoms in these two types of high temperature superconductors form three-dimensional sublattices.In fact,some solid hydrogen phases show obvious layered characteristics,while the Tc of hydride with layered structure is not very high.Are there layered hydrides with Tc of>200 K?So we performed a variety of crystal structure prediction techniques to screen tens of thousands of structures and predicted a high Tc superhydride HfH10 with a layered hexagonal P63/mmc phase at 300 GPa.Unlike covalent metallic H3S and clathrate-like LaH10,three H5 units in HfH10 form a layered "penta-graphene-like" H10 cluster in a"side-to-side" manner,while Hf atoms as electron donor and precompressor are located at the center of three H10 units.HfH10 is predicted to exhibit an extraordinarily high Tc of around 213-234 K at 250 GPa,which has the highest Tc among the transition metal hydrides.This "penta-graphene like" clustered H10 structure is also the first layered material with high values of Tc among superconducting hydrides.Furthermore,the"penta-graphene-like" structure is predicted to be a common motif in MH10(M =Zr,Sc,Lu)with Tc in a range of 134-220 K.It presents a new model structure(after H3S and the clathrate hydrides)with Tc higher than 200 K,where electronegativity,atomic radius,and valence electron configuration of the metallic element in these materials are found to play a critical role in stabilizing the "penta-graphene-like,hydrogen sublattice,and to fine-tune the superconductivity.The high Tc of the three models is related to the high H-dominant density of electronic states at the Fermi level and the strong electron-phonon coupling.The discovery of "penta-graphene like,,MH10(M?Hf,Zr,Sc,Lu)provides a new perspective for the exploration of high temperature superconductors.(2)Superconducting zirconium hydrogen compounds ZrH3 and Zr4H15 were predicted theoretically and synthesized experimentally under mild pressures.Zirconium hydrides are not only potential superconducting materials,but also traditional hydrogen storage materials.The limit of hydrogen storage is ZrH2.Is there a hydride with higher H content?Based on this,we have carried out extensive structural searches in the pressure range of 0-100 GPa for Zr-H system,and successfully predicted two hydrogen-rich compounds:Prn3n-ZrH3 and I43d-Zr4H15,with stable pressure ranges of 8-100 GPa and 4-100 GPa,respectively.Moreover,we predicted two ways to synthesize ZrH3 and Zr4H15:ZrHZ+H2?ZrH3/Zr4H15 and Zr+H?ZrH3/Zr4H15.In order to verify these theoretical results,we performed high pressure experiments in diamond anvil cell with ZrH2 +H2 and Zr+H2 as precursors,respectively Subsequently,the existence of these two high pressure hydrogen-rich phases was further confirmed by the results of synchrotron X-ray diffraction.The synthesis of ZrH3 and Zr4H15 successfully breaks the traditional hydrogen storage limit with high volumetric hydrogen storage densities of 163.7 g/L and 180.8 g/L.Electrical resistance and external magnetic measurements exhibit that the superconducting transition temperatures of ZrH3 and Zr4H15 at 40 GPa are 6.4 and 4.0 K,respectively,in good agreement with the theoretical results.We also compared the superconductivity of cubic ZrH3 with H3S,and found that the quite small contribution of H projected density of states to the total DOS at Fermi level and the large mass of zirconium atom inhibit the electron-phonon interaction of the system,resulting in relative low Tc.Our results provide a reference for the high pressure synthesis of new hydrogen-rich materials and the study of metal hydrides(3)The high temperature superconductivity of ternary clathrate-like hydride YCaH12 was determined.Theoretical calculations show that binary caged hydrides Im3m-CaH6 and Im3m-YH6 are potential high temperature superconductors.Y and Ca have similar atomic radii and electronegativity,theoretically,whether we introduce Y into CaH6 or Ca into YH6,it is possible to have high temperature superconductivity while ensuring that the H cage does not collapse.Therefore,first-principles calculations were preformed in Y-Ca-H system under high pressure.The clathrate Pm3m-YCaH12 was predicted to be stable in the pressure range of 180-257 GPa.Electron-phonon calculations show that YCaH12 has a large ? of 2.15 at 180 GPa,yielding a record-high Tc of 230 K.With the increase of pressure,the strength of electron-phonon coupling decreases,and Tc decreases accordingly.YCaH12 not only refreshes the record Tc of ternary hydrides,but also provides a new synthetic route for other ternary high Tc hydrides(4)The high pressure phase diagram and hydrogen storage capacities of ternary K-Si-H system are calculated.Experiments show that the ternary compound KSiH3 not only has a high hydrogen storage density,but also has a reversible reaction without disproportionation,indicating good hydrogen storage characteristies.Although another hydrogen richer compound K2SiH6 has been experimentally synthesized under high pressure,its stability at atmospheric pressure is still unknown,and the hydrogen storage performance is still unclear,which prompts us to study the K-Si-H system systematically.In our paper,four stoichiometries of K2SiH6,K2SiH8,KSiH7 and KSiH8 were predicted to be stable.Of these,K2SiH6 is stable in the explored pressure range Under ambient conditions,in addition to the stable phase Fm3m,there are two metastable phases P3m1 and p63mc for K2SiH6,in which hypervalent[SiH6]2-is formed K2SiH6 has a high hydrogen content of 5.4 wt%.The volumetric hydrogen storage density of Fm3m,P3ml and P63mc structures are 83.6,88.3 and 84.8 g/L,respectively And the corresponding dehydrogenation temperature with respect to KSi+K+H2 are 4.0/76.7,-37.1/33.7 and-14.9/62.6? theoretically.K2SiH6 is a potential hydrogen storage material,which can meet the standards of hydrogen storage materials in terms of hydrogen storage capacity and dehydrogenation temperature.Our results provide a useful guide for further experimental synthesis.