Perovskite-Related Copper Oxide Structures

As the copper ions are stable in square-pyramidal as well as square-coplanar and octahedral oxygen coordination, this allows the layered copper oxides to form a large 

In the structure Nd2 y zCeySrzCu04, the Cu ions have a fivefold oxygen coordination. However, with a smaller A cation that is stable in eightfold oxygen coordination, and a larger A cation in an AO layer, the family of copper oxide intergrowth stmctures that are related to the perovskite structure becomes extensive. Similar compounds may be represented by their sequence of successive (001) planes as follows  

The perovskite structure is very flexible, and the substitution of cationic positions A and B occurs over a wide range of compositions These compounds 

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The highest transition temperature for the "tungsten bronze" family was 7.7 K for an acid-etched (71) sample of composition Rb o g3W03. Certain researchers (62), after completing studies on cubic and tetragonai-II (semiconducting) bronzes, made the statement "It appears as though the (cubic) perovskite lattice is not favorable for superconductivity." This statement was made in 1965, prior to the major advances in copper oxides that are considered to have a related-perovskite structure. 

All the high Tc superconductors discovered so far, with one exception, contain weakly coupled copper oxide, Cu02, planes. The highest critical temperatures are found for cuprates containing a Group 2 metal (Ca, Ba, Sr) and a heavy metal such as Tl, Bi, or Hg. The structures of all the cuprate superconductors are based on, or related to, the perovskite structure. The one report (in 2000) of a non-cuprate high T superconductor is of surface superconductivity in Na WOs. The structure of NUxWOs is also based on the perovskite structure. 

A common feature of all the new ceramic superconductors is that they are cuprates, that is, they are complex copper oxides. The structure of YBCO is given in Fig. 19.3, which also shows that it is related to the perovskite structure (Fig. 4.17). Synthesis of YBCO is remarkably easy appropriate amounts of dry yttrium oxide (Y203), copper oxide (CuO), and barium carbonate (BaC03) are ground together into a fine, well-mixed... 

Horn et al. (1988) have compared the electronic structure of several superconducting copper oxide perovskites with closely related non-superconductors using several spectroscopic techniques including APS. Using XPS, a number of workers have... 

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