Crystal chemistry of Ruddlesden-Popper type structures in highT _c ceramic superconductors

Abstract

Similar structural patterns have been noticed in the systems La-Cu-O, La-Ni-O and Bi and Tl-containing superconducting oxides. The formation of Ruddlesden-Popper type layers (alternating slabs of rocksalt and perovskite structures) is seen in these oxides which is similar in many respects to what is seen in the system Sr-Ti-O. However, there are some significant differences, for example the rocksalt and perovskite blocks in new superconducting compounds are not necessarily electrically neutral, unlike in the Sr-Ti-O system. It, thus, becomes necessary to create oxygen vacancies in the basic perovskite (figure 1) structure of Bi-containing compounds, when the width of the perovskite slab changes on addition of extra Cu-O planes. Results of our atomistic simulations suggest that these missing oxygen ions allow the Cu-O planes to buckle in Bi-series of compounds. This is also supported by the absence of buckling in the Sr-Ti-O series of compounds and the first member of Bi-containing compounds in which there are no missing oxygen ions. We present additional results on the phase stability of polytypoid structures in La-Cu-O system and defect chemistry of compounds of La-Ni-O system.