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5.2 Iodine Intercalation into the Bi-Sr-Ca-Cu-O System

The most studied2-55 of these intercalated systems is I-Bi2Sr2Can-1CunO2n+4 which was first discovered in 1990.51 As iodine is added between the adjacent bismuth-oxide layers there is a shift from a staggered bismuth-oxide arrangement to an overlapping one with iodine centered between oxygen sites. Illustration 5.2 depicts the crystal structure of the iodine intercalated superconductor. This increase in symmetry allows for a change in the electronic properties along the c-axis.40 Before intercalation, the conductive behavior along the c-axis decreases as the temperature decreases prior to the transition temperature, showing semiconducting behavior. After intercalation, the conductivity increases with decreasing temperature, displaying metallic behavior. Although iodine intercalation causes a decrease in the superconducting transition temperature, the change is rather small. Before intercalation, samples display a transition temperature of ~85 K, while after incalation, the transition temperature is ~75 K.19 This lowering of the transition temperature by 10 K has been attributed to the overdoping of the material with iodine.13, 24, 28, 36, 43, 47 The decrease in transition temperature is minimal because the iodine, possibly in the form of I3-, is localized between the bismuth oxide sheets. Bismuth, like copper, can exist in a mixed-valence state and can partially shield the copper-oxide sheets from further doping. Raman studies reveal that iodine most likely enters the lattice as I3-. This behavior makes the iodine intercalated Bi2Sr2Can-1CunO2n+4 a highly oxidized system, which can possible be reduced when in the presence of material that is more easily oxidized.

Crystal structure of I-Bi2SrrCaCu2O8±x where the layers contain a) calcium, b) copper-oxide, c) strontium-oxide, d) bismuth-oxide, and e) iodine.

Illustration 5.2: Crystal structure of I-Bi2SrrCaCu2O8x where the layers contain a) calcium, b) copper-oxide, c) strontium-oxide, d) bismuth-oxide, and e) iodine.

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Updated on: April 15, 2010 8:26 PM