Study On The Chemical Pressure Effect In Transition Metal Based Quasi-Two-Dirmensional Compounds Containing Fluorite-Type Th₂N₂ Layer

Since the discovery of copper-based high-temperature superconductors in 1986,the association between unconventional superconductivity and antiferromagnetism has become a consensus in the community.A generally accepted picture is that the magnetic fluctuations that occur when the antiferromagnetic?AFM?state in the parent compound is suppressed play a key role in the superconducting pairing.Based on this idea,finding a suitable parent compound with AFM ground state and then trying to tune the AFM state has become an effective way to explore new superconductors.Since 2008,the research of the iron-based superconductors once again proved the effectiveness of the above strategy.In 2016,We successfully synthesized a new type 1111 iron-based superconductor Th Fe As N.Unlike most of the iron-arsenic superconductors,the compound exhibits superconductivity without AFM transition,though no carriers are introduced into the system by chemical substitution.Subsequent studies showed that a uniaxial chemical pressure along the c-axis accounts for the unique physical properties.This chemical pressure effect is widespread in other isostructural 1111-type compounds containing Th₂N₂ layers.Accordingly,the work of this thesis is divided into two parts:Firstly,we successfully synthesized a series of 1111-type manganese-based compounds Th Mn Pn N?Pn=P,As and Sb?containing Th₂N₂ layers.Though neutron diffraction data indicates the C-type AFM state has been established at room temperature,the magnetic susceptibility shows cusps at 36 K and 52 K respectively for Th Mn PN and Th Mn As N,denoting a second AFM transition.In addition,both susceptibility and specific heat suggest partial delocalization of the d-electrons.These properties are quite unique among the 1111-type Mn-based compounds,indicating that the chemical pressure reduces the stability of the AFM state in the Mn lattice.Secondly,we try to enhance the chemical pressure in the lattice by doping phosphorus on the arsenic site in Th Fe As N.Physical property measurements show that,upon P doping,three isolated superconducting areas are observed in the doping phase diagram Th Fe As_1-xP_xN.This is in stark contrast to the phase diagram of La Fe As_1-xP_xO,where only two superconducting areas were found.