Other Cuprates

In these and the other cuprate superconductors, the part of the structure that leads to superconductivity is the slab of Cu02 sheets. When more than one sheet is present, they are separated by cation layers, Q (usually Ca or Y) to give a sequence Cu02-(Q-Cu02) i, which forms the superconducting layer in the material. The index n is the total number of Cu02 layers in the phase, which is equal to the formula number of Cu atoms present (Fig. 8.5). 

These reagents are qualitatively similar to other cuprates in reactivity, but they are more stable than the dialkylcuprates. Because cyanocuprate reagents usually transfer only one of the two organic groups, it is useful to incorporate a group which normally does not transfer. The 2-thienyl group has been used for this purpose.9 In a mixed alkyl-thienyl cyanocuprate, only the alkyl substituent is normally transferred as a nucleophile. 

At exactly the same hydrostatic pressure an anomaly has been measured for the transition temperature and it has been attributed to a deformation of the Cu05 pyramids [31], This structural modification affects several phonons but mainly the double peak at -458-465 cm 1. The splitting of this mode must again be related with a phase separation mechanism, similar to the ones discussed above for the other cuprates. 

STM experiments suggested that in these materials at low temperatures one had intrinsic electronic spatial inhomogeneity. This conclusion is consistent with a proposal by Slichter on how one might reconcile the results of NMR experiments (a local probe) that seemed to require commensurate spin fluctuation peaks and INS experiments (a global probe) that show an incommensurate peak Slichter proposed that one is observing discommensuration in the 2-1-4 (and possibly other cuprates), with regions... 

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 crystal structure in italics is that observed at low temperature. Clearly the size of the trivalent ion is crucial in determining the preferred structure (r(La3 +) > r(Nd3 +)). Calculated lattice constants for these and other cuprates are compared with experiment in Table 10.1 overall, the agreement with experiment is very satisfactory. Even for Y2Cu2Os, which has relatively low symmetry with distorted Cu02 chains and Cu with an irregular five-coordination to oxygen, all three lattice constants are within 2.7% of the experimental values. 

This new reaction was shown to be applicable to various a,/3-epoxy ketoximes and to other cuprates. The products are easily dehydrated hence this method can be used to effect a-alkylation of a,/3-enones. 

The sol-gel method has been conveniently employed for the synthesis of 123 compounds, such as 63300307 and other cuprates. The sol-gel method... 

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