Thallium cuprates

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... 

While this chapter has focussed on synthesis of polycrystalline samples, other aspects of the bismuth and thallium cuprate superconductors are discussed elsewhere. An introduction to synthesis and crystal growth is given in Chapter 5, and a review of the crystal chemistry of the two families is presented in Chapter... 

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. 

All the cuprate superconductors discussed hitherto contain Ca, Ba or/and rare earth. Recently, superconducting thallium cuprates not containing Ca, Ba or even a... 

In the Tl2 xBa2Ca2+ Cu3O10 system, Kaneko et al. [79] have postan-nealed the as-synthesized x = 0.3 sample in an evacuated quartz tube at 750°C for 250 h and obtained a Tc of 127 K. Similarly, Liu et al. [80] have done similar postannealing treatments on x = 0.4 samples and obtained a Tc value of 128 K. This is the highest Tc value known for thallium cuprate superconductors. Usually, starting with little excess calcium helps in the phase formation of Tl-2223. 

Single crystals of thallium cuprate superconductors can be grown by the self-flux technique. 

The cuprate superconductors all have the same layered stracture that is sketched in Figure 11 for three thallium cuprates, which have one, two, and three copper oxide planes, that is, n = 2, and 3 in their chemical formula... 

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.

It was found that at pH > 11 in the region of low anodic overpotentials the product of thallium oxidation on an inert substrate represents the individual phase of a mixed-valence oxide which was previously unknown [352,253]. On a copper substrate, this same phase can be formed simultaneously with thallium cuprate, while at the higher overpotentials the amounts of both products in the deposit prove to be small due to the preferential formation of TI2O3 which proceeds at a high rate. At anodic overpotentials that are not too high, the rations of the amount of thallium cuprate to that of the mixed oxide in the deposits grown on copper correlates with the rate of active dissolution of copper [354], i.e., cuprate is preferentially formed at the higher pH. 

A similar model for the case of thallium cuprate is complicated by the large number of parameters that affect the state of the system, and also by the necessity to introduce two experimentally unknown quantities simultaneously into the model. However, taking into account the character of the pH dependences of all the processes that occur with the participation of thallium and copper, one can suppose that the shape of the thallium cuprate stability region would be close to that for the mixed oxide. That is, its width would increase with increasing concentration of copper ions (hydroxo complexes) within the crystallization zone. Although the crystallization process took place under a fortiori non equilibrium conditions, the results of the preparative analysis agree qualitatively with the concept of stability of the products. The interval over which formation of thallium cuprate on copper substrates occurs and the formation of the mixed oxide on platinum and carbon substrates can be observed is wider for higher values of the pH [352-354]. 

The simplest methods of HTSC analysis are based on the determination of the products of sample dissolution in acidic media. Potentiometric, amperometric, or coulometric titrations are frequently used (mainly for YBCO ceramics [525-527] and their analogs with other rare-earth elements [528, 529], and also for BSCCO [530]). We note particularly the method of potentiostatic coulometric analysis [531], which allows one to analyze thallium cuprate samples over a wide range of the Tl/Cu ratio, and also the method of flow-through coulometry for determining the effective valence of copper [532]. The polarographic determination of Cu content in the samples obtained by dissolving HTSCs in concentrated alkaline solutions with special... 

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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