Mercury cuprates

Those readers not familiar with superconductivity in organic materials may find the Tc values rather low. However, they are comparable to values for inorgaiuc metallic elements. Here is a list of some selected examples FcCNb) = 9.25 K, rc(Pb) = 7.20 K, rc(a-Hg) = 4.15 K, rc(Sn) = 3.72 K, Tc(Al) = 1.17 K, Tc(ri) = 0.40 K, etc. It is interesting to note that copper does not exhibit a superconducting transition. The highest known Tc values of any material correspond to the copper-oxide series with Tc 138 K as the absolute record for the thallium-doped mercury-cuprate compound. 

The mercury cuprates are diflScult to synthesize as pure phases, due to their low stability which originates form the dumbbell coordination of mercury. The difficulty in isolating pure phases increases with the number of copper layers. Moreover, only the barium layered cuprates HgBa2Ca jCu, 02m+2 are obtained, whereas the strontium homologous series could not be synthesized. The introduction of a rare earth into the Hg sites allows the anionic sites at the level of the Hg sites to be partially filled, and consequently the structure can be stabilized (for a review see Raveau et al. 1995a). 

The cuprates Hgi.5(Cu,Pr)o.5Ba2PrCui08 5 and Hg2 xM2,Ba2Pr2Cu20io-d were the first mercury cuprates with mercury bilayers that were synthesized (Martin et al. 1995, Huve et al. 1995). The first one has 2212 structure (fig. 28a), whereas the second one has the 2222 structure (fig. 28b). These oxides are not superconductors, but superconducting... 

The reaction has also been used to prepare 1,3-dilithiopropanes" and 1,1-dilithio-methylenecyclohexane" from the corresponding mercury compounds. In general, the equilibrium lies in the direction in which the more electropositive metal is bonded to that alkyl or aryl group that is the more stable carbanion (p. 228). The reaction proceeds with retention of configuration an Sgi mechanism is likely. Higher order cuprates (see Ref. 1277 in Chapter 10) have been produced by this reaction starting with a vinylic tin compound ... 

Tcr cuprates), whose superconductivity depends on subtle, phonon-free coupling between electrons. It is interesting that the highest known temperature Type-I superconductor, MgB2, shows a much larger isotope effect than does mercury (TCr (MgnB2) = 39.2 K, TCr (Mg10B2) = 40.2 K). 

Other addition reactions Additions involving carbon Copper(I) chloride, 85 Lithium bis(dimethylphenyl-silyl)cuprate, 161 Manganese(III) acetate, 171 Mercury(II) chloride, 175 2-(Phenylseleno)acrylonitrile, 244 Threophos, 298 Additions involving nitrogen Benzeneselenenyl halides, 26 Additions involving oxygen Bis(Tj5-cyclooctadienyl)ruthe-nium(II), 35... 

V,7V-Diethylcarbamoyl trimethylsilane has been prepared by the reaction of bis(trimethylsilyl) sulphide with bis(A,A-diethylcarbamoyl) mercury (Scheme 30)16. Silylation of the carbamoyl cuprate reagent derived from a lithium amide, by addition of copper(I) cyanide and subsequent exposure to carbon monoxide (1 atm), is also effective75,110. Poor to moderate yields of carbamoyl silanes may be isolated by treatment of lithium silylamides with carbon monoxide and methyl iodide, in a reaction sequence involving a nitrogen to carbon silyl shift in an intramolecular silylation (Scheme 31)111. 

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. 

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. 

Preparation of Superconductive Ceramics 17.3.10.2. High Temperature Superconductors 17.3.10.2.5. Mercury-Based Cuprates. 

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