Crystal Structure And Superconductivity Of 3d Transition Metal Co And Ni Based Compounds

Since the discovery of superconductivity,scientists have been searching for new superconductors with high critical temperatures(Tc).At present,only copper and iron-based superconductors are capable of Tc exceeding 40K under normal pressure.Copper and iron are the keys to high-temperature superconductivity.In the periodic table of elements,cobalt and nickel are located between iron and copper,but cobalt and nickel-based superconductors are rare.It has certain advantages for understanding the mechanism of iron and copper-based superconductivity.Based on the inorganic material database,a large number of candidate compounds that may have superconducting transitions are found through crystal structure design,and more than 120 compounds are synthesized by high-temperature solid-phase method,electric arc furnace melting,and other methods.In this dissertation,two new superconductors and one new compound were discovered.The specific research contents are as follows:We have discovered a new superconductor,LaCoSi.The crystal structure of LaCoSi is consistent with the "111" series LiFeAs.LaCoSi is the first Co-based superconductor with the Fe-based "111" structure.The electronic structure of LaCoSi has been studied by first-principles calculations,and its electronic structure is also quasi two dimensional.We jointly determined the superconducting transition of LaCoSi with Tc of 4K by electrical transport tests,magnetic tests,and specific heat tests.We fit the basic physical parameters of LaCoSi through the lowtemperature specific heat test.It is found that the structural distortion of CoSi4 tetrahedra in LaCoSi is smaller than that of other Co compounds with similar structures but lacking superconductivity,and the small structural distortion may be beneficial to the superconductivity in LaCoSi.The Tc of LaCoSi has been regulated by element substitution.The Tc is rapidly suppressed after replacing Co with Ni.Replacing Co with Fe is beneficial to the Tc.The best performance composition is LaCo0.75Fe0.25Si with Tc of 6K,and the upper critical field is also greatly improved.Hall resistance test shows that the carrier type in the samples substituted by LaCoSi and Fe and Ni is hole type.Fe substitution is equivalent to hole doping.It was found that the degree of structural distortion of CoSi4 tetrahedron is directly related to the Tc.The substitution of Ni makes the tetrahedral distortion of CoSi4 larger,while the substitution of Fe reduces the tetrahedral distortion of CoSi4.A new compound YCoSi and a compound LuNiSi with no reported physical properties were synthesized,and the crystal structures of YCoSi and LuNiSi were analyzed by SXRD.The variable temperature crystal structure,electrical transport,magnetic properties,and specific heat properties of these two compounds were systematically studied.Both YCoSi and LuNiSi are metallic and have no superconducting transition above 2K.Magnetic tests show that YCoSi and LuNiSi are paramagnetic at room temperature.Spin glass transition occurs at low temperature,the freezing temperature Tf of YCoSi is 50K,and the freezing temperature Tf of LuNiSi is 47K.The basic physical parameters of the two new compounds at low temperatures are fitted by specific heat.We found a new superconductor Mo2NiB2,and the crystal structure of Mo2NiB2 was studied by synchrotron radiation.The electrical transport test shows that Mo2NiB2 undergoes superconducting transition at around 4K,Mo2NiB2 is a typical type Ⅱ superconductor.Mo2NiB2 has been regulated in various ways by element substitution.The substitution of Mo sites and B sites can inhibit superconductivity,but the non-stoichiometric ratio of Ni sites can increase the Tc,and a small amount of Ni vacancies is beneficial to Tc.Hall resistance test shows that the carrier type in Mo2NiB2 series samples is electronic,and Ni vacancy reduces the electron carrier concentration,which is beneficial to the increase of Tc.