Superconductor defect structures

By atomic-resolution TEM imaging, simulation and nano-probe energy dispersive X-ray analysis, the detailed defect structure of j -type doping superconductor material was recently identified as composed of three layer Sr—O blocks, which may provide hole carriers into the Cu—O planes [8.26]. 

Many R-chalcogenides (R = Sc, Y, Lu, La) are superconductors. They crystallize in the rocksalt structure. Table 10.3 shows some examples for recently reported Te s, or the ranges of temperature, where superconducting transitions were observed. The Sc, Y and Lu monochalcogenides are known to be defect structures which can exist with an appreciable anion or cation deficiency (Hulli ger and Hull, 1970 Moodenbaugh et al., 1974). The Te s depend strongly on composition and are therefore not well-defined. 

NaCl structure, superconductor below 3.5 K), SotAss (defect NaCl structure, superconductor below 1.2K). The many important industrial applications of dilute alloys of As, Sb and Bi with tin and lead were mentioned on pp. 370 and 371. 

A Tripled-Perovskite — Relationship to the New High Tc Oxide Superconductor A tripled cell for the perovskite structure is shown in Figure 33. This structure is closely related to that of the new 90 K superconductor in the following way. The structure of the superconductor can be derived from a highly anion-defect, perovs-kite-type structure. The composition can be obtained as follows. 

The compound NCu02 (N = Ca0 86Sr014) is an insulator, but its structure, which is a simple defect perovskite made of layers (CuOa) sandwiched between layers (N), can be considered as the parent structure of a large family of superconductors. The sequence. ..(Cu0 )OiC(N)e o(Cu02)OjC. .. is in fact one of the building blocks of many compounds considered in this review. The refined parameters for NCu02 are given in Table 3. 

Figure 1 Relationship between the perovskite structure ABOs (left) and the defect-perovskite superconductor YBajCugOy (right). Metal atoms are shaded. Note the missing oxygen atoms in the latter drawing that result in formation of copper-oxygen sheets (above and below the Y atoms), and copper-oxygen chains (between the Ba atoms).

Point defects are also highly prominent in the Tl,Pb,Bi/Ba,Sr,-Ca/Cu/O superconductors. Cation vacancies frequently occur. Some T1 is found on Ca sites, and there is evidence for Ca on the Sr/Ba site. Some Bi is found on both Sr and Ca sites. Both oxygen interstitials and vacancies apparently can occur. Present evidence suggests that compounds with the ideal structures and compositions would not be metallic or superconducting. There are also strong indications that these materials at their ideal compositions are in fact too unstable to be prepared. 

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