8 - Cuprate Crystallographic Structures

Abstract

This chapter describes the structures of the high-temperature superconductors that are either tetragonal or orthorhombic. The prototype compound barium titanate, BaTiO3, exists in three crystallographic forms. The chapter illustrates the lattice constants and unit cell volumes for the three forms. All three alkaline earths, Ca, Sr, and Ba, appear prominently in the structures of the high-temperature superconductors. It is found that in the oxide superconductors Cu2+ replaces the Ti4+ of perovskite, and in most cases the TiO2–perovskite layering is retained as CuO2 layers, which is common to all of the high-temperature superconductors. It is observed that both the copper oxide planes and the chains contribute to the superconducting properties. The high density at the y3+ and Ba2+ sites and the lack of contours around these sites together indicate that these atoms are almost completely ionized, with charges of +3 and +2, respectively. The symmetry features of a body-centered superconductor by are explained using the example of T12Ba2CaCu2O8 in the chapter. All of the cuprates have structures with alternating conduction layers and binding layers stacked along the z direction. Copper oxide layers are either fiat or slightly puckered, in contrast to the other metal oxide layers, which are generally far from planar.