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

The most studied^2-55 of these intercalated systems is I-Bi₂Sr₂Ca_n-1Cu_nO_2n+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 Bi₂Sr₂Ca_n-1Cu_nO_2n+4 a highly oxidized system, which can possible be reduced when in the presence of material that is more easily oxidized.

Illustration 5.2:     Crystal structure of I-Bi₂Sr_rCaCu₂O_8±x where the layers contain a) calcium, b) copper-oxide, c) strontium-oxide, d) bismuth-oxide, and e) iodine.