CRYSTAL-STRUCTURE, CHEMICAL-COMPOSITION, AND EXTENDED DEFECTS OF THE HIGH-T-C (BI,PB)(2)SR2CAN-1CUNO(4+2N+DELTA) COMPOUNDS

This paper summarizes results obtained by high-resolution transmission electron microscopy on the crystal structure and microstructure of th e (Bi,Pb)(2)Sr2Can-1CunO4+2n+delta high-Tc superconducting oxides. The experimental basis for the work presented here are high-resolution st ructure images obtained at ultra-thin (3 nm) areas of carefully prepar ed transmission electron microscope (TEM) samples. The analysis was ca rried out on a 400 kV TEM equipped with a pole piece yielding 0.17 nm point-to-point resolution. From the images obtained the projected crys tal potential of the cations can be extracted directly, as confirmed b y detailed image simulation. Structural analysis of the oxygen sublatt ice remains an unsolved problem by high-resolution TEM (HRTEM), mainly because of the small scattering factors, and thus the contribution of the oxygen sublattice to the image contrast is small. The (Bi,Pb)(2)S r2Can-1CunO4+2n+delta phases are modulated structures that can be unde rstood as an average structure plus a superimposed displacement field. The crystal structure consists of BiO double layers and perovskite-ty pe cuboids (containing Sr, Ca, Cu, and O), which are sandwiched betwee n the BiO double layers. The displacement field can be directly analyz ed by HRTEM, and the largest displacement amplitudes of 70 pm were det ermined for the Bi atoms in the n = 1 compound. The chemical compositi on of the n = 2 and n = 3 compounds was determined by EDX in the TEM f or the cation sublattice. A significant (Ca + Sr) deficiency (approxim ately 10%) with respect to Cu was found. The (Sr + Ca)/Cu mole fractio n ratio was 1.31 for the Bi-2212 phase and 1.14 for the Bi(Pb)-2223 ph ase. The oxygen content cannot be determined by EDX in the TEM with th e accuracy necessary for a correlation with electrical and superconduc ting properties. The defect structure present in these materials, that is, intergrown lamellae with different crystal structures and equal o r different chemical compositions, stacking faults, and grain boundari es, is summarized. The importance of grain boundaries for understandin g and improving superconducting properties is emphasized. (C) 1995 Wil ey-Liss, Inc.