Structure And Property Modulation Of Several Layered Compounds Containing Fe Or Co

Layered compounds containing transition metals usually exhibit flexible structures and special physical properties in electricity,magnetism and other aspects.In this dissertation,we focus on the synthesis and characterization of several layered compounds containing transition metal Fe or Co,with special attention to their magnetic and thermoelectric properties.In an effort to suppress the phase separation in A_xFe_2-ySe₂ system,we synthesized three new layered compounds,studied their crystal structure and physical properties,and analyzed the hidden reasons behind the phenomena.In addition,we designed three novel layered compounds with intrinsic low lattice thermal conductivity,which shed light on searching for new thermoelectric materials.Finally,we studied certain layered compounds with complex Fe coordination environment,in order to further understand the relationship between the structure and physical properties.The main results of this thesis are summarized as follows:Firstly,three new layered compounds,namely,NaLiFeSe₂ with a CaAl₂Si₂-type structure,and KLiFeSe₂ and RbLi FeSe₂ with a ThCr₂Si₂-type structure,are discovered through replacing half of the Fe in A_x Fe_2-ySe₂ with Li.The introduction of Li at Fe sites can successfully eliminate the Fe vacancy and avoid phase separation,resulting in a homogeneous structure and composition in these materials.Through comparing of the structures of A_xFe_2-ySe₂ and ALi FeSe₂,it can be seen that the substitution of Li at Fe site compresses the layer spacing and elongates A-Se bondlength in ThCr₂Si₂-type structure.Thus,the substitution of Li at Fe site in Na_x Fe_2-ySe₂ would result in the elongation of Na-Se bondlength,leading to less overlap between Na and Se orbitals,which would destabilize the ThCr₂Si₂-type structure in NaLiFeSe₂,while the CaAl₂Si₂-type structure would be stabilized with shorter Na-Se bondlength and more overlap between Na and Se.We speculate that Na_x Fe_2-ySe₂ is at the border of ThCr₂Si₂-type structure and CaAl₂Si₂-type structure.Magnetic and resistivity measurements suggest an anisotropic spin-glass and narrow band-gap semiconductor ground state.The electrical conductivity behavior is well fitted by thermal activation model.The semiconducting behavior can be attributed to enhanced correlation effects.The discovery of these compounds would be instructive to further investigation of electronic correlations and magnetic structure in A_xFe_2-ySe₂materials.Secondly,we designed and synthesized three novel layered compounds CsA’MSe₂ with intrinsically low lattice thermal conductivities,namely,CsAgCoSe₂,CsAgFeSe₂ and Cs Li FeSe₂,with a space group of Acmm（No.67）.Their crystal structure can be regarded as a transformation of the ThCr₂Si₂-type structure with some atom arrangement ordering.Due to their complex layered structure,large atom weight,a low Debye temperature,as well as the disorder induced by the partial mix occupation,these compounds display low thermal conductivities.At room temperature,the lattice thermal conductivities of CsAgCoSe₂,CsAgFeSe₂ and Cs LiFeSe₂,are 0.34 W m^-1 K^-1,0.27 W m^-1 K^-1 and 0.45 W m^-1 K^-1,respectively,opening up new avenues for exploring new thermoelectric materials.Compared with CsAgFeSe₂ and Cs LiFeSe₂,CsAgCoSe₂ has a complex band structure,with a larger carrier effective mass,resulting in a higher Seebeck coefficient.The ZT value of CsAgCoSe₂ reaches 0.57 at 875 K.Finally,we report that Fe₃GeTe₂ can form a wide solid solution by substitution of As for Ge,providing an opportunity to tune the magnetic and electronic properties in this 2D material.The crystal structure,physical properties and electronic structure of iron-deficient solid solution Fe_3-yGe_1-xAs_xTe₂（0≤x≤0.85）are studied.We found that the Curie temperature can substantially change from 177 K to 33 K and resistivity decreases by about 30%with the arsenic doping x from 0 to 0.85.First principles calculations demonstrate that the elongation of Fe（1）-Fe（1）dumb-bells along c axis,rather than the carrier doping,is essentially responsible for the decrease of Curie temperature.Our study reveals the magnetism manipulation can be realized via modification of bondlengths in 2D magnetic materials.