Perovskite band structure, electronic conductivity

Figure 20. Electronic structure and transport in mixed conducting perovskites. (a) Band picture of electronic structure in the high-temperature metallic phase of Lai- r tCo03-(5. (Reprinted with permission from ref 109. Copyright 1995 Elsevier.) (b) Localized picture of electron/ hole transport in semimetallic Lai- 3r Fe03-(5, involving hopping of electrons and/or electron holes (depending on the oxidation state of iron).

Lithium can be inserted into the material up to at least 0.08 Li" " per formula unit. This level of intercalation is insufficient for the number of lithium and lanthanum cations to exceed unity and so the A sites of the perovskite structure still contain some vacancies at this stoichiometry. Whilst this intercalation process is reversible, experiments using this electrolyte in conjunction with a graphite electrode show that an irreversible oxidation process occurs. The reduction of Ti" " narrows the band gap and leads to electronic conductivity of 0.01 S cm at room temperature. This reactivity and electronic conduction would lead to a rapid discharge via short circuit of a stored battery and so makes these materials unsuitable for use as an lithium electrolyte in these applications. 

Both elements form dioxides, M02, that have distorted rutile structures in which there are M M bonds. Re03 is an important oxide but Tc03 is not known to exist. The structure of red Re03, shown in Fig. 18-D-l, is also adopted by Cr03, W03, and others and is very similar to the perovskite structure. Re03 has metallic type electrical conductivity due to delocalization of the Re 1 electrons in a conduction band of the solid. 

Last, ReOs has the octahedral framework of the SrTiOs structure minus the 12-coordinate atom in the center of the unit cell. However, the orbitals on this atom ate of such high energy (Sr electron configuration = 4s 4p 5s ) that they do not hybridize with the Ti 3d-02p bands. In the perovskite stmcture, this atom simply provides electrons to the system that can occupy the valence or conduction bands. Hence, there is little change to the band dispersion directly resulting from the presence of the A cation. 

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