Thallium 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. 

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... 

Fig. 5. Characteristics of thallium cuprate TlCuO(OH) electrosynthesized by anodic electrocrystallization [348] (a) structure calculated on the basis of X-ray crystal analysis (b) resistivity vs. temperature of the deposit on a copper substrate measured by the two-probe technique.

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. 

Very small deviations from oxygen stoichiometry can change drastically the superconducting properties of the thallium cuprates without destroying the structure. This is the case of the thallium cuprates for which a spectacular effect of oxygen... 

In contrast to the lead cuprates, most of the thallium cuprates do not require the presence of a rare-earth element for their stabilisation. The 40 K superconductor Tl j Prj.Sr2 Prj,Cu05 6 (Bourgault et al. 1989) seems to be the only one whose structure is stabilised by the presence of praseodymium. The isostructural phase TlSr2Cu05 5 could indeed not be isolated, but was only observed as a mixture with an unknown phase. In this 1201 structure (fig. 22), single perovskite layers [(Sr,Pr)Cu03]oo are intergrown with double rock-salt layers [(Tl,Pb,Sr)202]oo- On the other hand it is... 

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. 

Figure 2 Schematic representation of the ideal structures of the thallium monolayer cuprate superconductors.

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. 

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