Bismuth cuprates, structure

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. 

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... 

The fact that rock salt layers can adapt to perovskite layers suggests the possibility of formation of extended defects due to a variation of the thickness of these layers in the original matrix. Such extended defects are frequently observed by high resolution electron microscopy in bismuth cuprates and especially in thallium cuprates. Moreover the relationships between the two structural types and the fluorite structure makes that fluorite type defects are also observed. 

A particular feature of the bismuth cuprates deals wih the existence of incommensurate satellites on their electron diffraction patterns as shown for instance for Bi2Sr2CaCu208+8 (Fig. 34). This modulation of the structure is also observed on the single crystal X-ray... 

It is not necessary to introduce a rare-earth element to stabilize the structure of bismuth cuprates. Nevertheless, it is worth pointing out that in such phases calcium can be partly replaced by a rare earth. This is the case for the 2212 -type superconductor Bi2-.rPb2 Sr2Cai 2 Y2 Cu20g (Koyama et al. 1988) for wliich Tc remains constant, i.e. close to 85K, for 0 0.5, and then drops abruptly above x = 0.50. 

Among the high-temperature superconductors one finds various cuprates (i.e., ternary oxides of copper and barium) having a layered structure of the perovskite type, as well as more complicated oxides on the basis of copper oxide which also include oxides of yttrium, calcium, strontium, bismuth, thallium, and/or other metals. Today, all these oxide systems are studied closely by a variety of specialists, including physicists, chemists, physical chemists, and theoreticians attempting to elucidate the essence of this phenomenon. Studies of electrochemical aspects contribute markedly to progress in HTSCs. 

A number of chemical reviews of cuprate superconductors have included the bismuth and thallium families (11)-(14). Reviews focussing on the structural chemistry of these two series are also available (15),(16). On the thallium cuprates, an overview of structural studies has appeared (17), and a detailed review of... 

The layer-type structures and chemical nature of the constituents of the bismuth and thallium-based cuprate superconductors - notably the lone-pair stereochemistry of Bis+, variable valence of copper, and considerable exchange among some of the cation sites - combine to make structural non-ideality, nonstoichiometry, and phase intergrowth the rule rather that the exception in these families of materials. These features, as well as the probable metastability of the phases (and possibly all high-temperature oxide superconductors), also contribute to the difficulties typically encountered in preparing single-phase samples with reproducible properties and compositions. 

Most HTSC oxides have the structure of perovskite (though some of them have the spinel-type structure), which pertains to more than 35 structural classes [14], and includes more than a hundred typical unit cells [15]. Along with cuprates, certain bismuthates also exhibit HTSC properties. For fundamental studies of superconductivity in oxides, both the absolute Tc values and the variety of properties and structures are essential. Therefore, the titanium compounds with relatively low Tc values are also actively studied. 

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