A TEM study of the incommensurate modulated structure in Sr₂CuO_3+x superconductors synthesized under high pressure B. Structural model

This paper reports a detailed structural study of the incommensurate modulated phase in high pressure synthesized Sr₂CuO_3+x superconductors based on transmission electron diffraction, high resolution electron microscopy and simulations. Face-centered orthorhombic structure with an average commensurate lattice constant of about 5√2a_p×5√2a_p×c_p (p stands for the tetragonal La₂CuO₄-type structure) has been identified and an atomic model of this average structure is proposed. The modulated structure results from metal ion shear displacements with half sine and half cosine waves along 〈110〉_p directions. Analysis of the displacement fields suggests that the Cu ions have a half cosine wave displacement, which supports the existence of oxygen vacancies in the CuO_1+x layer proposed by a previous neutron diffraction study. If this modulated phase is indeed the superconducting phase our results call into question the requirement of intact CuO₂ sheets for superconductivity in high-T_c cuprates.