Thallium-Containing Superconductors

Tl2Ba2Cu06 (23) yielding a means of oxidizing the Cu02 sheets. Also, recent band structure calculations (discussed below) show that there is considerable overlap of the T16s-block bands with the Cu 3d x2-y2 bands providing a mechanism to transfer electrons from the Cu02 sheets to the Tl-O layers. 

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The discovery of thallium containing superconductors (4) was another important development. Several superconducting phases exist and consist of intergrowths of rock salt (TI-O) and perovskite layers. They have been reported with zero resistance and Meissner effect up to 125K, i.e., with the highest critical temperatures discovered so far. 

Two series of thallium-containing cuprate superconductors have been synthesized with the following ideal general formulas (10) ... 

In 1991, scientists at AT T Bell Laboratories discovered a new class of high-temperature superconductors based on fullerene, the allotrope of carbon that contains Cgo molecules (Sections 10.10 and 19.6). Called "buckyballs," after the architect R. Buckminster Fuller, these soccer ball-shaped Cgo molecules react with potassium to give K3C6o- This stable crystalline solid contains a face-centered cubic array of buckyballs, with K+ ions in the cavities between the Cgo molecules (Figure 21.16). At room temperature, K3Q,o is a metallic conductor, but it becomes a superconductor at 18 K. The rubidium fulleride, Rb C o, and a rubidium— thallium-Cfio compound of unknown stoichiometry have higher Tc values of 30 K and 45M8 K, respectively. 

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

To date, many of the ceramic superconductors contain copper and share a common structural feature copper and oxygen atoms bonded together in planar sheets. In YBCO superconductors, the Cu—O planes are widely separated. In bismuth superconductors, the Cu—O planes occur in "sandwiches" consisting of two closely spaced sheets separated by a layer of group 2 ions. These sandwiches are separated from one another by several layers of bismuth oxide. In the thallium superconductors, the Cu—O planes are stacked in groups of three, like triple-decker sandwiches. 

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