Microstructure Study And First-principles Calculation Of A_xFe_ySe₂?A=K,Rb,Cs,Tl? Iron-based Superconductors

Iron-based superconductors have a rich physical phenomenon,and phase transitions exist in many systems.In this paper,the microstructure of iron-based superconducting system is studied by transmission electron microscopy.The electronic structure of iron-based superconducting material K0.7Fe1.77Se2 is studied and utilized by the combination of density functional theory and electron energy loss spectroscopy.MS software simulates the influence of pressure on the lattice structure and electronic structure of AxFeySe2(A=K,Rb,Cs,Tl).The conclusions reached are as follows:(1)The K0.7Fe1.77Se2 superconducting material was analyzed by transmission electron microscopy.The experimental results show that the sample structure is a tetragonal structure.The order of the Fe vacancies in the Fe-Se layer produces a superstructure.This superstructure can be modulated with a wave vector.q1=1/5(130)and q2=1/4(130)indicate that the two sets of modulated wave vectors are mirror-symmetrical.(2)To simulate the electronic structure of K0.7Fe1.77Se2 material,it is found that the density near the Fermi level is mainly related to the 3d orbit of Fe and the 4p orbit of Se.In the low energy loss spectrum,about 4.5 eV corresponds to the transition of the 3d orbit of Fe to the 3p orbit of K.The transition of 3p orbital electrons corresponding to K to the 3d orbital of Fe and the 3d orbital of Se to the 3d orbit of Fe are obtained.The peak of 22.6 eV corresponds to the plasma peak.27.7 eV corresponds to the transition of the 3p orbital electron of K to the 3d orbit of Fe.(3)The effect of Core-hole on the EELS of Fe in K0.7Fe1.77Se2 materials was investigated.Core-hole could obtain the results consistent with the experiment.The effect of U on the electron energy loss spectrum of Fe in K0.7Fe1.77Se2 material was studied by the same calculation method.The calculated results were in good agreement with the experimental results when U=5 eV.The increase of the L3 peak of Fe is cleaved,indicating that U affects the transition of electrons in the 2p3/2 orbital of Fe to the 3d3/2 and 3d5/2 orbitals of Fe.(4)Simulate the influence of pressure on the lattice constant and the electronic structure of AxFeySe2(A=K,Rb,Cs,Tl)materials.The K0.7Fe1.77Se2 is in the range of 0.5 GPa to 7.0 GPa,the lattice constant a is changed by about 3.4%,the lattice constant c is reduced by about 7%,and the lattice constant c is increased by 2.5%at 9.0 GPa.Two phases appeared.The distance between K-K is increased by 1.2 A;the distance between Fe-Fe is reduced by 0.36 A;and the distance between Fe-Se layers is reduced by 0.85 A.The study also found that RbFe2Se2 has a similar change at 11.0 GPa,CsFe2Se2 at 9.0 GPa,and TlFe2Se2 at 11.0 GPa.