High-pressure Studies Of Typical Lanthanide Superhydrides

Room temperature superconductivity has been a very challenging research front for nearly a century.According to the BCS theory of traditional superconductors,the Debye temperature of the material is inversely proportional to the square root of the mass,so according to the theory,solid hydrogen will become a high-temperature superconductor or even a room-temperature superconductor.Pressure is the most effective way to achieve metallic hydrogen.But the current experimental pressure has reached about 495 GPa,and there is still no direct evidence to realize hydrogen metallization.Higher experimental pressure is needed for further research,but the realization of ultra-high pressure above 500 GPa in experiments and the corresponding characterization techniques show great challenges.Until 2004,Ashcroft proposed that non-hydrogen atoms with larger volumes can be introduced into the hydrogen crystal lattice to form hydrogen-rich compounds.Due to the effect of chemical pre-pressing,hydrogen-rich materials are potential high-temperature superconductors that are much easier to become metallization than pure hydrogen.In 2014,the original prediction on the T_c of H₃S could exceed 200 K under high pressure,which broke the superconducting record maintained by copper-based oxides.Later,this prediction was confirmed in high-pressure experiments by our group and the laboratory from Germany and Japan,which inspired a new wave about the research on hydrogen-rich compounds.Recently discovered cubic LaH₁₀-is a remarkable high-temperature superconductor?T_c²50 K?which creates a new era of superconductivity.Therefore,hydrogen-rich compounds have become the best candidate system to solve the century-old problem of room temperature superconductivity.The theoretical prediction results showed that lanthanum hydride has a unique H-cage configuration,and high-temperature superconductivity is closely associated with clathrate structures of hydrogen.Combined with the theory of precompression on hydrogen,selecting elements with heavier atomic weights and more valence electrons in non-hydrogen elements has higher possibility to form hydrides with higher hydrogen proportion.In order to continue to search for potential high-temperature superconductors,we synthesized the typical lanthanide metal elements and hydrogen in the conditions of normal temperature and high pressure or high temperature and high pressure by using in situ synchrotron X-ray diffraction and in situ electrical measurements in DACs combined with first-principles calculations.We investigated the structure and properties of the new lanthanide hydrides and explored the role of metal atoms in the structure and superconductivity of lanthanide hydrides.The superconducting regularity changes of some lanthanide hydrides were summarized.The innovative results as followed:1.Superconducting lanthanide praseodymium superhydrides were obtained for the first time.By using H₂ and NH₃BH₃ as hydrogen resource,we synthesized several novel metallic superhydrides F4?3m-PrH₉ and P6₃/mmc-PrH₉,two trihydrides Fm3?m-PrH₃and P4/nmm-PrH_3-?in the pressure range 0–130 GPa.The superhydrides F4?3m-PrH₉and P6₃/mmc-PrH₉ have similar hydrogen cage configurations as fcc-LaH₁₀ and P6₃/mmc-CeH₉.Resistance measurements showed that the synthesized mixture of cubic and hexagonal PrH₉ demonstrated a resistance drop around 9 K in high pressure,indicating possible superconducting transitions in both PrH₉ were below 9 K.Further calculation results showed magnetic order and electron-phonon interaction coexist in a very close range of pressures in praseodymium hydrides which may have an effect on the low superconducting transition temperature.Present results on Pr superhydrides show that superconductivity declines along the La-Ce-Pr series,while magnetism becomes more and more pronounced.2.An antiferromagnetic neodymium superhydride was obtained.The heavier neodymium reacted with H₂ generated from ammonia borane NH₃BH₃?AB?in high temperature and pressure condition.From 90 to 140 GPa,we synthesized three compounds I4/mmm-NdH₄,C2/c-NdH₇ and P6₃/mmc-NdH₉.The resistance measurements of synthesized NdH₉ demonstrated that there is no superconducting transition at 5 to 300 K over 100 GPa,indicating a possible superconducting transition may happen at⁴-5 K.Ab initio calculations of magnetization indicated that I4/mmm-NdH₄,C2/c-NdH₇ and P6₃/mmc-NdH₉ all possess antiferromagnetic?AFM?collinear[112],[144]and[23???]orders with weak anisotropy at low temperatures respectively.This is the first example of hydrides with pronounced magnetic properties.Compared with lanthanum hydride,the increased number of f electrons in neodymium pronounced the magnetic properties,at the same time may suppress the traditional superconductivity of neodymium hydride based on electron-phonon coupling.It is further clarified that magnetism is an important factor to affect the traditional superconductivity in hydrides.3.Europium superhydrides with strong magnetism were obtained.Our research on lanthanide hydrides was continued with europium hydrides with more f electrons.Using the previously proven technology of high-pressure hydride synthesis by laser heating of mixtures Eu and ammonia borane at 110 GPa,we have synthesized superhydrides P6₃/mmc-EuH₉ and a small amount of F4?3m-EuH₉.Analysis of magnetization showed a significant increase in the total magnetic moments of europium hydrides compared with neodymium hydrides.We found that the most stable magnetic configuration of cubic and hexagonal EuH₉ exhibit anti-ferromagnetism and ferromagnetism,respectively.