Structure And Property Of Layered Compounds Containing Fe

Layered compounds containing transiton metal have a great variety of structure type and physical property.Iron based superconductors with layered structure has sparked widespread interest ever since its discovery.However,the superconducting mechanism of high temperature superconductor still needs to be solved.Considering the competing relationship between superconductivity and antiferromagnetism,the main purpose of this thesis is to investigate the relationship of magnetism and superconductivity in iron based superconductors.Besides,considering the supression of magnetic order in magnetic frustration compounds,several new geometrically frustrated compounds are explored.The main research results are summarized as follows:1,A series of Cu_1-xLi_xFeAs compounds with site mixing between Cu and Li were prepared,and a continuous solid solution from CuFeAs to LiFeAs was achieved for the first time.As Li doping level increases,the transition temperature of antiferromagnetism is suppressed.At x=0.5,no sign of antiferromagnetism can be observed.With further increasing the doping level of Li,superconductivity starts to emerge.A phase diagram is constructed to show this evolution,which features no coexistence region between superconductivity and antiferromagnetism.This behavior shows that antiferromagnetic CuFeAs can be regarded as a parent compound to the observed superconductivity by equivalent doping,which is different from the cases with other FeAs-based superconductors.Structural analyses indicate that the anion height of Fe₂As₂ tetrahedral layer plays a crucial role on the physical properties.Moreover,first principles calculations indicate the simple Fermi surface nesting picture might be not applicable to Cu_1-xLi_xFeAs.2,Through moderate temperature sintering,we firstly report a FeSe-based compound CsFe_4-?Se₄ with fairly low content of alkali-metal,which is closely related to alkali intercalated FeSe superconductors while exhibits distinct features.It does not undergo any phase separation or antiferromagnetic transition.Powder neutron diffractions,electron microscopy and high-angle annular-dark-field images confirm that CsFe_4-?Se₄ possesses an ordered Cs arrangement as?2×?2 superstructure,evidencing a B-centered orthorhombic lattice with a space group of Bmmm.DFT calculations and transport measurements reveal a novel Fermi surface geometry with two electron-like sheets centered on?point and intermediate density of states at the Fermi level comparing with the value of FeSe and the superconducting A_xFe₂Se₂.The absence of superconductivity might be due to the insufficient carrier concentration in the FeSe plane.To solve this problem,increasing the Fe contents by several methods have been tried.Besides,doping effects on Fe site and Se site are also investigated.3,We report here the properties of three new quasi-2D triangular lattice antiferromagnets FeAl₂Se₄,Co_0.66Al₂Se_3.53 and Ni_0.61Al₂Se_3.55,which show highly magnetically frustrated characters.Powder X-ray diffractions demonstrate that they possess identical space group of P-3m1.For FeAl₂Se₄,its Curie-Weiss magnetic susceptibility above 150K reveals significant antiferromagnetic correlations with an Curie-Weiss temperature?_cw of-200K.The extracted local moment is consistent with a total spin moment S=2.Despite the large S value and the large|?_cw|,FeAl₂Se₄shows remarkable low-temperature behaviors incompatible with conventional classical magnets.Dc and ac susceptibilities indicate a spin-freezing behavior at about14K.However,no long-range or short-range magnetic order was observed by polarized neutron powder diffraction down to 3.5K.Measurements of the magnetic specific heat show a T² power-law behavior below the freezing temperature.Our results suggest the coexistence of spin glass and spin liquid behavior in FeAl₂Se₄.For Co_0.66Al₂Se_3.53,Analyzing the susceptibility data reveal a Curie Weiss temperature of-216 K,and a spin freezing transition temperature of 4.5 K,giving a frustration index f=-?_cw/T_f?48.Ni_0.61Al₂Se_3.55 possesses a Curie Weiss temperature of-62 K with no sign of spin freezing transition down to 2 K.Considering the gradual decrease of the spin size from FeAl₂Se₄ to Co_0.66Al₂Se_3.53 and Ni_0.61Al₂Se_3.55,we speculate spin size has a significant influence in the frustration index of these materials and is responsible for the more obvious spin liquid behavior.