Study On Substitution Effect Of The Same Valence Elements In ThFeAs1-xPnxN(Pn=P,Sb)System

In 2008,the discovery of superconductivity in La Fe As O_1-xF_xsystem marked the birth of the iron-based superconductors after the cuprates,sparking a wave of exploration in high-temperature superconductors.Similar to cuprate,iron-based superconductors can be seen as alternating stacking of antifluorite-type Fe₂X₂（X=P,As,Se）conducting layer and a charge reservoir layer along the c-axis.The parent compound of iron-based superconductors generally exhibits antiferromagnetic（AFM）spin density wave state.When charge carriers are introduced by element substitution,the AFM state is suppressed,leading to superconductivity.Befroe long,scientists realized that when part of the As^3-ions in the Fe₂As₂layer are replaced by P^3-ions,superconductivity can also be induced though no charge carriers are nominally introduced.Knowing that the P^3-ion is smaller than that As^3-,the substitution always leads to cell contraction,and is therefore called the chemical pressure effect.On the other hand,when As^3-is replaced by larger isovalent Sb^3-,superconductivity does not occur while the AFM order is further strengthened.The above isovalent substitution effect shows that the d-p orbital hybridization in iron-based superconductors has a significant impact on superconducting properties.In 2016,our reported the first Zu Cu Si As type iron-based nitride Th Fe As N.The compound exhibit superconductivity at 30 K without AFM spin density wave state.We attributed the superconductivity to the intrinsic chemical pressure along the c-axis.In this dissertation,we conduct the isovalent substitution in the Th Fe As_1-xPn_xN（Pn=P,Sb）system to regulate the chemical pressure.The result show that the isovalent substitution effect in Th Fe As_1-xPn_xN（Pn=P,Sb）is significantly different from other 1111-type iron-arsenides,suggesting that the Th₂N₂layer in Th Fe As N has an additional charge transfer effect on the conducting layer.In addition,our synthesized a new compound Th₂Cu₄As₅.The lattice of this material can be seen as alternating stacking of a fluorite-type Th2As2 layer and a antifluorite type Cu₄As₃conducting slab.Resistance and magnetization measurements show that Th₂Cu₄As₅undergoes superconducting transition at 5.2 K,marking the discovery of the first superconductor based on a double-layered antifluorite-type conducting layer.At the same time,we noticed that the magnetic susceptibility,resistivity,and specific heat capacity of this compound all exhibit anomalies around 50 K,suggesting the presence of a metal-metal phase transition in the system.More over,the Hall coefficient changes sign at around 50 K,suggesting the metal-metal phase transition is associated with multiband scenario.