BASED CUPRATE SUPERCONDUCTORS

Preparation of Bismuth- and Thallium-Based Cuprate Superconductors 

Within weeks after superconductivity was observed above 77 K in the Bi-Sr-Ca-Cu-O system, Sheng and Hermann (3) reported a record-high Tc value of 120 K for a Tl-Ba-Ca-Cu-O phase. The results were quickly confirmed (4)-(9) and the existence of a large family of thallium-containing superconducting oxides was revealed (10). 

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 

Two series of thallium-containing cuprate superconductors have been synthesized with the following ideal general formulas (10)  

It is common practice to refer to these compounds using numerical acronyms corresponding to the metal stoichiometries. The decoding scheme and Tc values are collected in the following table (13)(21)-(23) the lead-substituted phases are included but not generally referred to with acronyms. 

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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. 

The primary difficulty in preparing the thallium-based cuprate superconductors lies in the toxicity and volatility of the reactant Tl2Os and its decomposition products. Above 600°C, the following redox-vaporization process is well under way and would lead to substantial loss of reactant in an open system, although the... 

The following sections will outline specific methods for the synthesis of Bi- and Tl-based cuprate superconductors. Because the synthetic methods and historical evolution of the compounds are different, the bismuth and thallium families are described separately. 

One of main thrusts in organic-perovskite hybrids has been in the preparation of stable delaminated perovskite sols as the building blocks of nanocomposite materials. A well-known process for exfoliation is to weaken the attractive interaction of the layers through intercalation of bulky organic components. A representative example is exfoliation of bismuth-based cuprate superconductors (Figure 14.12). °3... 

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