A Review of Intercalation in Heavy Metal Iodides

This review outlines the work to date concerning the mysteries of both the large optical effects and the bonding mechanisms involved with intercalation in heavy metal iodide systems. Such a system exhibits unusual shifts of the optical band edge to higher energies upon intercalation. The shift depends on which host−guest combination is used. Several models have been developed to describe this system. The bonding mechanism for this system is not clear because some data appear to support a model based on a polarization interaction between the guest and host material whereas other data support a charge transfer or quantum confinement model. There appear to be several different systems being formed by intercalation of the same substances that have different stoichiometry. This riddle may be solved by investigations of the particular methods of intercalation.