Microstructure and microchemistry of defects and interfaces in Tl₂Ba₂Ca₃Cu₄O₁₂, TlBa₂Ca₄Cu₅O₁₃ and (Tl,Pb)Sr₂Ca_n-1Cu_nO_2n+3(n=2,3)oxide superconductors

Abstract

Microstructural and microchemical observations from high resolution electron microscopy and STEM microanalysis are presented on local structure, extended defects and interface structures in bulk superconducting oxides with compositions: Tl2Ba₂Ca₃Cu₄O₁₂ ('2234'), TlBa₂Ca₄Cu₅O₁₃ ('1245') and in the related (Tl,Pb)Sr₂Ca_n-1Cu_nO_2n+3 (with n=2,3). Interfaces are directly imaged with near-atomic resolution to reveal the microstructure and microcrystallography. The real space images show that the grain boundary regions (including at triple points), contain intervening amorphous layers, which are several nanometers (nm) in extent in the (Tl,Ba) systems but are much narrower, ≈2-3 nm in extent in the (Tl,Pb) systems. EDX of the interface regions shows the presence of copper (oxide) but no Tl or Pb. There are nonstoichiometric and nonequilibrium intergrowths in the (Tl,Ba) systems which contribute to the variation in local chemistry. There is evidence for 6-perovskite layer defect intergrowths in the '1245' thallium preparations. End-on edge dislocations, and stoichiometric deficits are observed. The (Tl,Pb) systems have smoother microstructures, relatively fewer intergrowths and no incommensurate modulations and have transition temperatures (T_c) close to the corresponding (Tl,Ba) materials. In contrast to the thallium studies, the grain boundaries in the bulk '2212' BiSrCaCuO system are clean, and essentially atomically flat, but contain large misorientations.