Perovskites perovskite-related copper oxide

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... 

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 ... 

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... 

Another closely related family of superconductors is represented by the formula TlBa2Can 1Cun02n+3 (n = 1, 2, 3, 4, 5). They contain single layers of T1 and O atoms that separate the perovskite-like Ba-Cu-Ca-O slabs (24)(25)(26)(27) (Figure 6). Distortions in the Tl-O sheets are also found in these compounds (26)(27). Note that if these phases were stoichiometric, copper would always have a formal oxidation state of greater than two. Therefore, the chemical composition of this homologous series allows the existence of holes in the copper-oxygen sheet. 

Perovskite oxides have also been studied as support of some other active phases. The promoting effect of the SrTiOs perovskite on the activitjr of impregnated copper for the catalytic combustion of soot was reported. The higher activity of Cu/SrTiOs in comparison with CU/AI2O3 was attributed to the improved redox properties of copper originated by the Cu-support interactions, which improve the catalytic oxidation of NO to NO2. This appears to be related to the presence of weakly bound oxygen in this sample, since the copper species present on the Cu/SrTiOs catalyst were more readily reduced than those in other inert supports [26],... 

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