Crystal structure analysis of Co-based layered cuprates CoSr2(Y1−xCax)Cu2O7+δ by transmission electron microscopy techniques

Crystal structures of Co-based 1212-type layered cuprates, CoSr2(Y1−xCax)Cu2O7+δ (x = 0, 0.4; δ = 0, 0.3) have been investigated by means of transmission electron microscopy (TEM). For the non-superconductive CoSr2YCu2O7.0 parent phase (x = 0, δ = 0), electron diffraction (ED) patterns revealed superlattice reflections due to an ordered arrangement of two kinds of chain of CoO4 tetrahedra that are mirror images of each other, along the b direction. In the non-superconductive CoSr2(Y0.6Ca0.4)Cu2O7.0 phase (x = 0.4, δ = 0), the superlattice reflections were found to be streaked along the b∗ direction, indicating that the partially disordered arrangement is induced by aliovalent CaII-for-YIII substitution. In the superconductive CoSr2(Y0.6Ca0.4)Cu2O7.3 phase (x = 0.4, δ = 0.3), the superlattice reflections are not seen anymore, suggesting that the distinction of the two CoO4 chains has disappeared. Furthermore, high-resolution TEM observation revealed that the introduced excess oxygen atoms interrupt the regular shift of the chain position along the c direction.