Structure of High T c Oxide Superconductors
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MRS BULLETIN/JANUARY 1989
consists of a perovskite cube with Ba or K on the corners, Bi in the center, and oxygens in the faces, as shown in Figure 1. Being a truly cubic system, it cannot be described as a layered compound. The superconducting phase and structure is present for x>0.25 with the highest Tc occurring for compositions close to a structural phase transition into a nonsuperconducting phase at x-0.25. It is important to highlight a major structural feature of this superconductor by comparing it with the long-known BaPb,. ; t Bi I 0 3 c o m p o u n d (T c ~13 K). 3 Under the generally believed assumption that the conductivity occurs in the strongly b o n d e d t h r e e - d i m e n s i o n a l Bi-O sublattice, the doping of the Bi site is more detrimental to superconductivity (i.e., lower Tc) than the doping in the Ba site. The nonmetallic parent comp o u n d c o m m o n to both s t r u c t u r e s , BaBi0 3 / can be m a d e metallic (and superconducting) by doping with Pb on the Bi site or K on the Ba site. The K-doped compound, Ba,.rKTBi03, w h e r e the Bi-O s u b l a t t i c e r e m a i n s intact, has a substantially higher Tc. "Single-Layered" L a 2 . , R r C u 0 4 (R = Ba, Sr, Ca) The discovery of superconductivity near 30 K in La 2 . t Ba I Cu0 4 by Bednorz and Mtiller was the beginning of the unprecedented activity in this field. 4 The structure of this compound is identical to that of the undoped orthorhombic L a 2 C u 0 4 s t r u c t u r e or its h i g h temperature tetragonal polymorph. It can be described either as a stacking of perovskite cubes or as a layered compound. For the rest of this article, it is convenient and more consistent with the electronic properties to discuss its structure as a layered superconductor. The structure of the superconducting
compositions was first studied by x-ray and neutron diffraction.5'6 It consists of o n e p l a n e of Cu a t o m s w i t h four strongly bonded oxygen neighbors in a square-planar arrangement at a distance of 1.89 A, and two additional weakly bonded oxygen neighbors above and below the Cu atom at a distance of 2.43 A, as shown in Figure 2. If one considers the weakly bonded oxygen atoms to be associated with La rather than with Cu (consistent with calculations of the electronic structure), the layered nature of the structure is clearer. The square-planar C u 0 2 planes are intercalated by two LaO layers (or, equivalently, a corrugated double LaO layer). Successive C u 0 2 planes are displaced from one another by 1/2 of the unit cell along the tetragonal [110] direction, resulting in a doubling of the unit cell along the c axis. The highest Tc in this family of compounds (T c =37 K for R=Sr) occurs for compositions near A: =0.15. The substitution of the divalent R cation for trivalent La introduces carriers into the conduction band associated with the C u 0 2 planes. At this composition, the structure is tetragonal, I 4 / m m m at room temperature with two formula units per unit cell. However, at low temperature the structure is orthorhombic
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