Bismuth cuprate

The electron microscopy studies of the superconductive cuprates show that the different families differ from each other by the nature of their defect chemistry, in spite of their great structural similarities. For example, the La2Cu04-type oxides and the bismuth cuprates rarely exhibit extended defects, contrary to YBa2Cu307 and to the thallium cuprates. The latter compounds are characterized by quite different phenomena. 

Curiously, the thallium cuprates often exhibit intergrowth defects contrary to the bismuth cuprates, whose structures are very similar. 

The first important point is that satellites in incommensurate positions are observed in all superconductive bismuth cuprates. They are directed along the [100] or [010] directions and can appear along two perpendicular directions due to the existence of domains at 90°, characterized by a perfect coherent interface (Figure 30a). 

The volatility of the reactants is a concern in some solid state syntheses. This may be a slight problem in bismuth cuprates synthesized at high temperatures because Bi2Os has an appreciable vapor pressure at these temperatures (i.e. 900-950°C) however, chemical analyses of samples of bismuth-based superconductors before and after reactions at temperatures up to 900°C indicate no detectable loss of bismuth. This problem is much more severe in the case of thallium chemistry. 

Figure 7.30 The ab plane resistivity in cuprate superconductors as a function of temperature (a) Bismuth cuprate (2201) (b) Laj gjSro jjCuO (c) Bismuth cuprate (2212) (d) YBajCujO,.

The sol-gel method has been conveniently employed for the synthesis of 123 compounds such as YBa2Cu307 and the bismuth cuprates. Materials prepared by such low-temperature methods have to be annealed or heated under suitable conditions to obtain the desired oxygen stoichiometry as well as the characteristic high Tc value. 124 cuprates, lead cuprates and even thallium cuprates have been made by the sol-gel method the first two are particularly difficult to make by the ceramic method. Coprecipitation of all the cations in the form of a sparingly soluble salt such as carbonate in a proper medium (e.g. using tetraethyl-ammonium oxalate), followed by thermal decomposition of the dried precipitate has been employed by many workers to prepare cuprates. 

Bismuth cuprates of the general formula Bi2(Ca,Sr)n+1Cu 02 +4 possessing an orthorhombic structure and containing two rock-salt type layers of BiO constitute an important family of superconductors, with the n = 2 and the n = 3 members showing Tcs of 90 K and 110 K respectively (figure 10). The n = 1 member of the formula Bi2+2.Sr2 a.Cu06 (without Ca) shows a maximum Tc of around 20 K. The n = 1 member containing calcium has been reported, but it does not appear to be... 

An interesting series of bismuth cuprates in terms of the variation of the Tc as well as the hole concentration with composition is provided by Bi2Sr2Ca1 a.Lna.Cu208 where Ln = Y or rare earth (Rao et al. 19906). The electrical resistivity data show a metal-insulator transition in the normal state with change in x (figure 12). The Ta as well as the nb show a maximum at a composition of x = 0.25 (figure 13). Note that when Ca is fully substituted by Ln, the material becomes a non-superconducting insulator. Hole concentration in these bismuth cuprates is readily determined by Fen-Fem redox titrations. 

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