Synthesis And Study Of Novel Cerium-based Hydrogen-rich High-temperature Superconductors At Relatively Low Pressures

Superconductivity,as a macroscopic quantum state,has extraordinary physical properties.Zero resistance and diamagnetism are two major characteristics of superconductors.Since the first experimental discovery of superconductivity in 1911,many superconducting materials have been found in elementary substance,alloy,copper-based,and iron-based systems.The superconducting critical temperature is continuously refreshed.The T_c of copper-based superconductors reaches 133 K at ambient pressure,and can be increased to 164 K under pressure.In recent years,the highest T_c of iron-based superconductors has reached 56 K.However,the research progress of traditional high temperature superconductors is relatively slow.In 2001,the highest T_c found in Mg B₂ is only 39 K,which is much lower than that of unconventional superconductors.According to BCS theory,hydrogen-based superconductors may be high-temperature superconductors and even room-temperature and"hot"superconductors because of the high Debye frequency of hydrogen.High pressure research is an effective means to regulate the structure and properties and obtain new materials.In the past decade,the successive discovery of H₃S and La H₁₀ under high pressure has raised the superconducting transition temperature by nearly 100 K again,triggering a research upsurge in hydrogen-based high-temperature superconductors.The experimentally discovered high-temperature superconducting hydrogen compounds all require pressures of more than one million atmospheres,which significantly limits the synthesis and characterization of related compounds and is not conducive to future applications.Hence,we investigated some Ce-based polyhydrides and discovered several new high-temperature superconducting hydrogen-rich compounds in binary Ce-H and ternary La-Ce-H systems,respectively.The hydride superconductors with T_c over 100 K have been first achieved under megabar pressures.We also found some factors affecting the stability and superconductivity through careful analysis of these polyhydrides.In addition,we studied the alkaline earth metal Ba-H system and discovered the superhydrides with extraordinary stability.The specific innovative results of this thesis are as follows:1.Firstly,we developed the four-electrode photolithography method applied to the diamond anvil and realized the in-situ electrical measurement at above megabar pressures.We successfully synthesized two high-temperature superconducting cerium polyhydrides:P6₃/mmc-Ce H₉ and Fm-3m-Ce H_10,and investigated their superconducting properties based on the developed method.The T_cof P6₃/mmc-Ce H₉reached 57 K after synthesis at 88 GPa and was greatly enhanced during compression to 120 GPa.The cubic Fm-3m-Ce H₁₀phase was first discovered in the experiment with the T_cof 115 K at 95 GPa.In addition,we studied the Ce-D system for the isotope effect using pure deuterium（D₂）and deuterated ammonia borane（ND₃BD₃）.The obtained isotope coefficient is 0.49,which accords with the prediction from BCS theory.This was the first successful attempt to find high-temperature superconducting hydrides at lower pressures.Anomalously,low pressures of stability of cerium superhydrides make them appealing for studies of superhydrides and for designing new superhydrides with even lower pressures of stability.2.We noticed the structural similarity between the binary La-H and Ce-H systems which show high critical temperature and low stabilization pressure,respectively.We chose the La-Ce alloy with La:Ce ratio of 3:1 as the precursor to study the superconductivity of La-Ce-H ternary polyhydrides.We successfully synthesized the P6₃/mmc-（La,Ce）H_9-10 at pressures lower than 130 GPa.Strikingly,it has a T_c of 176 K and H_c2（0）of 235 T at 100 GPa.For comparison,we also studied the La-H system at the same pressure region.The superconductivity of La H_x can also be preserved to below megabar pressure with the T_c of 100 K at 78 GPa.Compared with the theoretical calculation results,we believe that the random substitution of metal atoms in La-Ce-H and the distortion of hydrogen cage structure increase the configuration entropy of the system and stabilize the high-temperature superconducting phase at lower pressure.These results show how better superconductors can be engineered in the new hydrides by the addition of alloy-forming elements.3.Barium atoms with large atomic radius and strong reductiveness in alkali earth metals may be conducive to molecular hydrogen dissociation and chemical pre-compression.We synthesized the barium superhydride Cmc2₁-Ba H₁₂subsequent to our previous work on lanthanide polyhydrides.Ba H₁₂ was evidenced to have a distorted cubic structure by in-situ synchrotron XRD and theoretical calculation.The hydrogen atoms exist in the form of H₁₂ chains that are composed of the H₂ and H₃^-units.The conductivity of Ba H₁₂ comes from the hydrogen layer.Ba H₁₂ can be stabilized to the pressure as low as 75 GPa,about one-fifth of what molecular metallic hydrogen requires.In-situ high pressure measurements show that the superconducting transition temperature is 20 K at 140 GPa.Our results proved that barium is a favorable component for synthesizing the compounds with high hydrogen content.It provides an important reference for the synthesis and research of superhydrides and superconducting hydrogen-rich compounds.