Microstructure Investigation In ACr₃As₃,Fe_1+yTe_xSe_1-x Superconductors And Bi_4.2K_0.8Fe₂O_9+δ Multiferroics

The microstructure of functional materials,such as atomic occupancy,defects,structural inhomogeneity and phase separation,has an evident effect on physical properties.Therefore,the microstructure characterization is crucial for the fully understanding of the underlying physical mechanism.Transmission electron microscope(TEM)shows significant advantages in functional material microstructure investigation due to its high-spatial resolution and comprehensive characterization capability.In this thesis,the microstructures of several typical superconductors,multiferroic materials were systematically characterized by electron microscopy to explore the correlation between microstructure and physical properties.It mainly includes the following parts:1.ACr3As3(A=K,Rb)single crystals were prepared by chemical cation deintercalation from their A2Cr3As3 counterparts.The microstructure of ACr3As3 was systematically characterized by scanning and transmission electron microscope.The nominal ACr3As3 crystals generally exhibit irregular nanoscale ACr3As3 phase domains accompanying with an amorphous phase,observing along the[001]direction.The Cr3As3 chains rotation and subsequent structural relaxtion contribute to the major part of the shrinkage of sample volume and the amorphous crack formation.EDS mapping reveals that the amorphous phase is As-deficient and both A and Cr elements are homogeneously distributed.In addition,we found some small K2Cr3As3 phase and intermediate state in the KCr3As3 matrices.The number of K+cations in the intermediate region can be less than nine while keeping the arrangement of Cr3As3 chains similar to that of the K2Cr3As3 phase.Our microscopic results provide detailed description of the microstructural evolution during the phase transformation:the Kion deintercalation of K2Cr3As3 prefers to start from one channel.In the intermediate region,only eight K+columns exist around each Cr3As3 chain and the arrangement of Cr3As3 chains are almost unchanged.With further K-ion deintercalation,the remained K+ cations redistribute and drive the rotation of the Cr3As3 chains.The structure with seven K+ columns has never been observed,suggesting that such a structure is highly unstable.The concentration of K+cations and the associated local strain are key factors modulating the arrangement of the Cr3As3 chains.Furthermore,detailed atomically resolved high-angle annular dark field(HAADF)images analysis of the area where the K2Cr3As3 and KCr3As3 phases coexist shows that the intermediate region usually spans over one or two Cr3As3 chains with modulated rotation angles.2.The room-temperature multiferroic Bi4.2K0.8Fe2O9+δ nanobelts is composed of alternating rock-salt and perovskite layers along the c direction.The dielectric rocksalt layers,which are alternatively sandwiched between perovskite layers,can be used to manipulate the lattice distortions of the perovskite layers and the corresponding properties.The modulation structure of Bi4.2K0.8Fe2O9+δ taken along the[100]direction was systematically characterized by TEM,and the displacement components of c and b directions can be fitted by a periodic function.The quantitative measurements show that the waves are transverse along c direction and longitudinal along b direction,respectively.The transverse modulation waves in the four adjacent Bi layers are in phase and the wave amplitudes of the peripheric layers are nearly the same and slightly different from those of the two medium layers.The displacement direction has a mirror symmetry between the two medium layers.The longitudinal waves are also in phase with each other and the amplitudes of the peripheric waves are smaller than those of the medium ones.The EDS mapping shows the layered distribution of Bi,Fe and K,indicating the K ions are mainly located in a layer sandwiched between two adjacent Fe layers,instead of randomly occupying the perovskite sites with Bi ions.3.Interstitial iron(Fe2)concentration has a significant effect on the physical properties of Fe1+yTe and Fe1+yTexSe1-x samples.The microstructure of Fe1+yTe and Fe1+yTexSe1-x samples were systematically characterized by TEM.An evident phase separation has been observed in Fe1+yTe samples;in some areas,the interstitial Fe2 atoms are randomly distributed,while in other areas,a locally ordered phase can be clearly seen and gives rise to a modulated structure of wave vectors q=1/8(3,0,4).Comparing to the Fe1+yTe samples,Fe1+yTe0.5Se0.5 samples contain much less interstitial Fe2 atoms,and no local ordering states has been observed in the tested samples;this microstructure feature difference could relate to the higher superconducting transition temperature in Fe1+yTe0.5Se0.5.In addition,detailed atomically resolved HAADF images analysis shows that the Se substitution happens randomly in Fe1+yTe0.5Se0.5 samples,and chemical inhomogeneity can be often observed.