Alkaline-earth cuprates

The use of a prereacted precursor is the typical approach to the preparation of ceramics of the Tl-containing superconductors (S). For example, to prepare the 120K T1 phase, H2Ba2Ca2CusO10, or 2223-T1, an alkaline earth cuprate precursor which is a mixed phase sample of overall stoichiometry, Ba2Ca2CusOx is prepared by standard ceramic techniques. An intimate mixture of the precursor and a stoichiometric... 

Copper(m), which is isoelectronic with Ni11, occurs in several crystalline compounds, but it is unstable in solution. Thus NaCu02, KCu02, and other alkali and alkaline-earth cuprates, obtained by heating the appropriate oxide mixture in oxygen, are stable as dry solids but dissolve with decomposition in aqueous base the Cu111 survives no more than a few seconds in solution.43... 

The enthalpies of reactions between alkaline-earth cuprates M2CUO3 (M = Ca, Sr) and hydrochloric acid were measured in a hermetic swinging calorimeter at 298.15 K. The M2CUO3 samples were prepared by solid-phase synthesis from calcium or strontium carbonate and copper oxide. The standard enthalpies of formation obtained for the cuprates were discussed and compared with previous experimental and assessed values by Monayenkova and coworkers [102]. 

The entire situation changed when Bednorz and Muller (1986) discovered 30 K superconductivity in a cuprate of the La-Ba-Cu-O system. This broke the apparent 23 K barrier for the T. Since 1986, a variety of cuprates have been synthesized and characterized with the maximum reaching 150K. The cuprates are unusual in many ways and do not seem to follow the BCS model. We shall discuss the cuprates at length later in Chapter 7. Alkali and alkaline-earth derivatives of Cgo (buckminsterful-lerene) exhibit superconductivity with a maximum of 35 K. We shall examine these... 

Not only compositions containing all the HTSC metal components, but also simpler subsets, may be considered as the precursors. Thus, by a combined technique [189], Ba-Ca-Cu films were obtained by electrodeposition and then thallium was introduced from the vapor phase in the course of simultaneous oxidation. In [190, 191], it was shown that reproducible preparation of Bi-Pb cuprates can be achieved when three-component precursors are deposited and the alkaline earth cations are then introduced before annealing. It is practically impossible to provide reproducible deposition of five-component precursors. Two-stage electrosynthesis of HBCCO [200] included the intermediate annealing of a Ba-Ca-Cu deposit followed by mercury electrodeposition on the resulting oxide substrate. 

Recently, HTSC properties were found for certain new complex cuprates of alkaline-earth and rare-earth metals [237,238]. Electrosynthesis of these compounds by analogy with the formation of barium cuprate films [228,229] appears to be possible. 

The products of degradation during polarization of HTSC electrodes have not been studied in much detail [44,287,293,476,478], Upon cathodic polarization of cuprates, the reduced forms of copper oxides [44,476], and sometimes metallic copper [293], are formed at considerable rates these processes are photosensitive [453], During anodic polarization, the formation of alkaline-earth compounds on the HTSC surface is accelerated [287,476]. 

The structurally simplest cuprate superconductors are doped La CuO (the T or T/0 structure), doped Nd CuO (the T stmcture) and oxygen-deficient YBa GUjO,. At room temperature, La CuO adopts a distorted version of the K NiF structure, in which the CuO octahedra are Jahn-Teller lengthened along the c-axis (Figure 4.7). At higher temperatures the Jahn-Teller distortion is lost and the material adopts the undistorted K NiF stmcture. The room temperature insulating stoichiometric compound is transformed into a superconductor by replacement of some of the La " cations with the alkaline earth cations Ba, Sr + or Ca. The stmcture of Nd CuO ... 

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... 

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