Thallium -oxide-based superconductors

The thallium-based superconductors with the general formula Tl A2Ca iCu 02 + +2, where w = 1 or 2 n = 1-5 A = Ba,Sr, are of interest because of their high superconducting temperatures. Unlike their rare-earth-based relatives, the Tl-based copper oxides are thermally unstable phases and can be difficult to prepare in the pure phase. 

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. 

Since the discovery of high temperature superconductivity (T > 30 K) in the La-Ba-Cu-O system hundreds of high temperature superconducting oxides have been found, with the record Tc of 164 K in Hg-1223 under high pressure. According to their major chemical difference, these superconductors can be classified into six major categories K2Nip4-type cuprates, rare-earth-based cuprates, bismuth-based cuprates, thallium-based cuprates, mercury-based cuprates, and other cuprates. Details of every superconductor are not presented, but at least one typical composition of each major type will be discussed in detail. Interested readers can read the relevant literature to get specific information on other systems. 

The recently discovered thallium-based high-temperature superconductors received extraordinary attention because their transition temperature is above that of liquid nitrogen, 100-120 K, and these materials seem to be both stable in air and resistant to moisture. Different thallium cuprates are prepared by melting (920-950°C) appropriate amounts of mixed powders of TI2O3, BaO/SrO, CaO, and CuO. The oxides have the idealized formulas TlA2Ca , iCu ,02 +3 (A = Sr, Ba) andTl2Ba2Cam-iCum02m+4- No deviation from... 

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