Theoretical Studies On The Superconductivity Of Hydrogen-rich Compounds Under High Pressure

Since the discovery of superconductors,it has attracted the attention of many scientists for more than a hundred years due to its superior electromagnetic properties and wide application prospects.It is also people's long-term goal to find practical high-T_c superconducting materials.As the first microscopic theory to successfully explain the superconductivity,BCS theory provides a theoretical basis for scientists to explore traditional high-T_c superconductors.Based on the BCS theory,a high Debye temperature and electron-phonon coupling strength are essential to improve the T_c of phonon-mediated superconductors.Thus,solid hydrogen is first considered a candidate material for high temperature or even room temperature superconductor since it is the lightest element.But hydrogen is an insulator at ambient pressure.As we know,high pressure can change the physical properties of materials,and it's considered an effective means of insulation-metal transition.Back in the 1930s,scientists discussed the possibility of hydrogen transforming to metal at P>25 GPa.In 1968,Ashcroft proposed that metallized hydrogen would be a promising high-T_csuperconductor.However,much studies show that the pressure required for the metallization of solid hydrogen is quite high,and it is quite challenging to achieve superconducting transformation in solid hydrogen.Ashcroft extended the original predictions of very high-temperature superconductivity of metallic hydrogen,he later suggested that hydrogen-rich materials,such as group IVA hydrides,maybe have superconductivity at lower pressures due to hydrogen has undergone the“chemically precompression”effects by other elements,and hydrogen-rich materials can also be candidates for high-temperature superconductors.Therefore,as an alternative to metallic hydrogen,much effort has devoted into the studies of metallization and superconductivity of compressed hydrides.In this paper,the crystal structure prediction technology combined with first principles calculation method are used to study the high-pressure phase diagram,crystal structures,thermodynamic and dynamic stability,electronic properties and superconductivity of binary La-H system and ternary N-Si-H system.The details are as follows:(1)Crystal structures of La-H compounds at 0-150 GPa has been systematic studied using the CALYPSO crystal structure prediction method and software.Some thermodynamically stable phases in the La-H system have been found by calculating the formation enthalpy relative to the elemental La and H.We further investigated the electronic properties and superconductivity of the stable compounds I4/mmm-La H₄ and P-1-La H₅.The EPC calculations show that the superconducting transition temperatures of La H₄ and La H₅ were 74-85 K and 42-53 K at 150 GPa,respectively.The EPC parameter was mainly contributed by H-H vibration mode,which plays a decisive role in improving the superconducting temperature.(2)The crystal structure of N-Si-H system at 300 GPa was explored by CALYPSO crystal structure prediction software,and the high-pressure phase diagram and electrical properties of N-Si-H system were studied by first principles calculation.Theoretical study found that the metastable phase NSi H₁₁(space group:P2₁/m)with the lowest positive enthalpy contains the large number of H₂ units.ELF calculations reveal the weak covalent feature between H₂ units.The EPC calculations show that NSi H₁₁ is a potential high-temperature superconductor with T_c of?110 K at 300 GPa.The interaction between N atom and Si-H system plays a role in improving the superconductivity.Our current results provide a new superconducting candidate and guidance for further experimental synthesis for ternary hydrides under high pressure.