Stability and lattice pressure in complex perovskite superconductors

Abstract

Spatial and electronic criteria for stability of complex perovskite superconductors are analyzed. We describe rules according to which complex perovskite materials are not likely to be obtained when simple perovskites can be built from some of the elements in question. For materials such as YBa 2Cu 3 O x , one finds two qualitatively different Cu stackings, namely a symmetric one ( sCu between Ba planes) and an asymmetric one ( aCu stacked between Ba and Y). aCuO adjusts to the available space by buckling, while sCuO is planar. This results in increased lattice pressure on sCu , leading to oxidation beyond Cu 2+, i.e., to hole creation, with increasing x, which is initially localized on sCu . We suggest that this is the origin of an independent modification with T s ∼ 60 K. When holes can penetrate to aCu , T s ∼ 90 K. These arguments can be generalized to include materials such as Bi 2Sr 2Ca n−1 Cu n O x . Strategies for searching for new materials are outlined.