Chapter 188 Crystal chemistry of superconducting rare-earth cuprates

Abstract

Publisher Summary This chapter discusses the relationships between crystal structures extended defects and superconducting properties of rare-earth-based cuprates. Rare-earth elements stabilize different layered structures involving thallium, lead, and mercury, owing to their large size and also to their trivalent character allowing more oxygen to be incorporated. In the case of lead cuprates, several structures have been generated because of the introduction of rare-earth elements. The first of these oxides is the 68 K superconductor Pb 2 Sr 2 Ln l-x Ca x Cu 3 O 8 with x≈0.50, whose structure consists of double pyramidal copper layers [Cu 2 O 5 ]∞ interleaved with calcium and yttrium ions, as already observed in YBa 2 Cu 3 O 6 , associated with single rock-salt-type layers [(Sr, Pb) 1 O]∞ similar to those observed in La 2 CuO 4 -type oxides. The rock-salt layers are themselves connected through layers of monovalent copper, with Cu(I) in two-fold coordination, showing a great similarity with YBa 2 Cu 3 O 6 . The hypothetical oxide Pb 2 Sr 2 YCu 3 O 10 can be considered, whose structure consists of double oxygen-deficient perovskite layers and single perovskite layers intergrown with single rock-salt-type layers [(Pb,Sr) 1 O]∞. Such an oxide corresponds to a double intergrowth, that is, an intergrowth of the La 2 CuO 4 -type structure (single perovskite layers) with the La 2 CaCu 2 O 6 -type structure (double perovskite layers). The elimination of the oxygen atoms of the basal planes of the CuO 6 octahedra, belonging to the single perovskite layers of “Pb 2 Sr 2 YCu 3 O 10 ”, leads to the 68K superconductor.