Investigations Of Superconductivity And Microstructures In CsBi₄Te₆ And CsPb_xBi_4-xTe₆ Systems

The narrow-gap semiconducting CsBi₄Te₆ and orthorhombic CsPbxBi_4-xTe₆（0.3≤x≤1.0）materials are investigated by（scanning）transmission electron microscopy（STEM/TEM）,Quantum Design Physical Property Measurement System（PPMS）,Quantum Design SQUID magnetometer（MPMS）,Powder X-ray diffraction and scanning electron microscopy（SEM）.The correlations between the microstructures and physical properties have been analyzed.Element substitution effects in CsBi₄Te₆ material have been investigated,and experimental measurements clearly reveal the presence of bulk superconductivity in the orthorhombic CsPbxBi4-xTe6（0.3≤x≤1.0）materials.Moreover,La-Sr-Eu-Cu-O materials have been synthesized to investigate the minimum Sr content in the superconductors.1.CsBi₄Te₆ presents a notable low-dimensional layered structure,such as,structural channels and Cs vacancies,which often appear in this material.Microstructural analyses based on aberration-corrected scanning transmission electron microscopy（STEM）observations demonstrate that low dimensional Cs_xBi₄Te₆ materials,known to be a novel thermoelectric and superconducting system,contain notable structural channels that accurately appear at the cross points of Bi-Bi bond layers and Cs atomic layers,and go directly along the b axis,with the size of approximately 0.5 nm × 1 nm.The structural channels can be partially filled by atom clusters depending on the thermal treatment process.We successfully prepared two series of Cs_xBi₄Te₆ single-crystalline samples using two different sintering processes.The Cs_xBi₄Te₆ samples prepared using an air-quenching method show superconductivity at approximately 4 K,while the Cs_xBi₄Te₆ with the same nominal compositions prepared by slowly cooling are nonsuperconductors.Moreover,atomic structural investigations of typical samples reveal that the structural channels are often empty in superconducting materials;thus,we can represent the superconducting phase as Cs1-y Bi₄Te₆ with considering the point defects in the Cs layers,and the monoclinic angle（β）is 102.3°.In addition,the channels in the nonsuperconducting crystals are commonly partially occupied by triplet Bi clusters,and the monoclinic angle（β）is 100.5°.2.Structural and physical properties of CsBi₄Te₆ show up notable changes following replacements of Bi by Pb,the structural alterations have been also investigated in relevant systems.We have synthesized CsPbxBi4-xTe6（0.3≤x≤1.0）,Cs_xPbBi3Te6（0.90≤x≤1.10）and CsBi4SexTe6-x（0.2≤x≤1.2）.Experimental measurements clearly reveal the presence of bulk superconductivity in the orthorhombic phase of thermoelectric CsPbxBi4-xTe6（0.3≤x≤1.0）,with the highest Tc=3.1 K.Structural study using aberration-corrected STEM/TEM reveals a rich variety of microstructural phenomena in correlation with the ordered structure related to Pb and chemical inhomogeneity in all superconducting samples.Temperature dependences of electrical resistivity,magnetic susceptibility,and specific heat have consistently demonstrated that the superconducting transition in CsPb0.3Bi3.7Te6 occurs at Tc=3.1 K,with a superconducting volume fraction close to 100% at 1.8 K,which may due to the ordered structure related to Pb with a specific modulation wave vector q=a*/2+c*/1.35 on the a-c plane and the structural instability,such as local distortion and defects.3.La-Sr-Eu-Cu-O materials have been synthesized and their structure and physical properties have also been preliminary investigated.Temperature dependence of electrical resistivity demonstrates that the minimum Sr content in the superconductors is 0.15.