Intercalation band-filling

Fig. 7.15 Band filling in an intercalation model according to Friedel s (1954) notion of screening. The upper panel shows the position of the bands at various degrees of filling the lower panel shows the corresponding values of the electron chemical potential (Fermi energy).

The filling control approach has even been applied to some nanophase materials. For example, the onset of metallicity has been observed in individual alkali metal-doped single-walled zigzag carbon nanotubes. Zigzag nanotubes are semiconductors with a band gap around 0.6 eV. Using tubes that are (presumably) open on each end, it has been observed that upon vapor phase intercalation of potassium into the interior of the nanotube, electrons are donated to the empty conduction band, thereby raising the Fermi level and inducing metallic behavior (Bockrath, 1999). 

This situation, clear in the case of more ionic structures, is less stringent in graphite intercalates where, presumably, there is electron transfer to (in the case of alkali-metal intercalates) or from (in the case of metal halide intercalates) the half-filled conduction bands of the graphite layers (produced by overlap of the 7t orbitals). Similarly, the periodicity requirements are less stringent for the alternating composite layers of layer silicates with complex intralayer and interlayer charge balance. 

Doping of fullerenes proceeds similar to the doping of polymers. Electron donors are intercalated to the rather large octahedral and tetrahedral interstitial sites of the fee lattice and fill the originally LUMO derived band of the semicondeting pristine compound with electrons. The resulting materials are fullerides, salts of the fullerites. Basic information on the structure and properties of fullerites and fullerides can be obtained from the special... 

The stractures of the alkali metal fullerides may be considered to be intercalation compounds of the fee lattice of the Ceo host formed by filhng the interstitial sites. Superconductivity has been investigated in detail for the alkali metal and mixed alkah metal phases of composition AgCeo where the conduction band is half filled. Critical temperatures as high as 31.3 K (in Rb2CsCeo) have been observed. A clear correlation between unit cell size and Tc is observed, and it has been suggested that the conduction bandwidth controlled by the separation between the molecules is the dominant factor in determining Tc. 

Using band structure nomenclature for the host solids allows a much more detailed discussion of the intercalation reaction. The electronic transfer is influenced not only by the number of empty levels but also by the structure of the band itself, i.e, the number of electrons in orbitals, or density of states. A pseudo-plateau in the potential variation of a discharge curve may be related to the filling of a zone of high density of states. This pseudo-plateau does not mean a two-phase region but occurs because the difference in energy of electrons in the alkali metal, i.e., before intercalation, and in the host, i.e, after intercalation, remains quite constant. 

With the stoichiometry ZrSej 94, zirconium diselenide is a semiconductor with an empty tjg band ( 16.4.3.1). The LijZrSej 94 intercalation compounds show a phase transition when x reaches 0.40. A continuous filling of the octahedral sites of the van der Waals gap (see Fig. 1, 16.4.3.1.) is observed over the whole composition domain (0 < X < 1) according to a classical CdIj-NiAs transition. Electrical, electrochemical, magnetic, NMR and EPR measurements affirm that it is a purely electronic transition. Below X = 0.40 the electrons are localized on zirconium centers, reducing Zr ions Zr above x = 0.40 a metallic behavior is observed. 

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