High Pressure Synthesis And Superconductivity Of Calcium-based And Lanthanide Polyhydrides

Owing to unique zero resistance and complete diamagnetic properties,superconductivity has great potential application and research value in many fields.Searching for high temperature superconducting materials is an important research direction in condensed matter physics.Solid metallic hydrogen,in case of light atomic mass and high debye temperature,is the most promising candidate of room temperature superconductor.Pressure can effectively compress the atomic spacing,making the material transform from gas to liquid and then to solid state.If the pressure is high enough,usually the band gap of the material will decrease and then the bands will overlap,so the material will transition to metallic state.Hydrogen is gaseous and insulating under ambient circumstance,and pressurization is an effective means to obtain metallic hydrogen.Although huge progress has been made in the research of metallic hydrogen,direct evidence of successful obtaining metallic hydrogen is still missing due to the limitation of experimental technology.In 2004Ashcroft proposed that non-hydrogen elements in hydrides have a"chemical precompression"effect on the hydrogen sublattice,the minimum pressure required for metallization of hydrides is much lower then hydrogen,and the properties of hydrides are similar to metallic hydrogen,therefore hydrides are expected to be a good candidate of high temperature superconductors.In this thesis,the alkali earth metal Ca、the lanthanide metal Lu and Yb are heated with ammonia borane（as hydrogen source）under high pressure,respectively,using the diamond anvil cell technology and in-situ high-pressure laser heating technology.And binary metal polyhydrides(Im3mm-Ca H₆、Pm3mn-Lu₄H₂₃（Fm3mm-Lu H₁）、Pm3mn-Yb₄H₂₃)are successfully synthesized,Then in-situ high-pressure electrical experiment and structural characterization are conducted,and the research results are as follows:（1）High temperature superconducting calcium polyhydride Im3m-Ca H₆ with H₂₄ cage structure was successfully synthesized for the first time.Using metal Ca and ammonia borane,calcium polyhydride samples were successfully synthesized under high temperature and high pressure circumstance（160-205 GPa and 2000 K）.The results of in-situ high-pressure synchrotron radiation X-ray diffraction experiments indicated that the calcium polyhydride sample is Im3m-Ca H₆,which is the first successful synthesized metal polyhydrides with hydrogen cage structure apart from rare earth metal hydrides and actinide hydrides.In in-situ high-pressure electrical experiments,the resistance of Im3m-Ca H₆ suddenly decreases and reaches0 eventually,and the transition temperature gradually decreases with the increase of magnetic field strength,which confirms the superconductivity phenomenon.At 160GPa,the T_c of Im3m-Ca H₆ is 210 K,which is the highest T_c among alkaline earth metal hydrides found so far.While pressure is released,Im3m-Ca H₆ can be stably for at least 140 GPa and maintain superconducting properties.（2）High temperature superconducting lutetium polyhydrides were synthesized successfully for the first time.Lutetium polyhydride samples were synthesized by heating metal Lu and ammonia borane under high pressure（181-218 GPa,2000 K）.In-situ high-pressure synchrotron radiation X-ray diffraction results showed that the samples contain Pm3mn-Lu₄H₂₃ and Fm3mm-Lu H₁.In-situ high-pressure electrical experiments showed that the resistance of lutetium hydride samples suddenly decreases to 0,and the transition temperature gradually decreases with the increase of magnetic field strength,which confirms the superconductivity phenomenon.At218 GPa,T_c is about 71 K,which is the highest T_c among the binary late lanthanide hydrides.（3）Ytterbium polyhydride Pm3n-Yb₄H₂₃ was synthesized successfully for the first time.Ytterbium polyhydride was synthesized from metal Yb and ammonia borane at about 179 GPa and 1500 K.In-situ high-pressure synchrotron radiation X-ray diffraction results showed that the ytterbium hydride sample is Pm3n-Yb₄H₂₃.In in-situ high-pressure electrical experiments,the resistance of ytterbium polyhydride samples suddenly began to decrease sharply at 179 GPa and 11 K,and the transition temperature decreased gradually with the increase of the magnetic field strength.It is very likely that the superconducting transition phenomenon occurred.Pm3mn-Yb₄H₂₃ has the highest hydrogen content in the Yb-H material synthesized by experiments.The hydrogen ytterbium stoichiometric ratio exceeds 3:1 for the first time,and it is likely that superconductivity has been observed in the Yb-H material system for the first time.