Electron intercalated compounds

Among the alkali metals, Li, Na, K, Rb, and Cs and their alloys have been used as exohedral dopants for Cgo [25, 26], with one electron typically transferred per alkali metal dopant. Although the metal atom diffusion rates appear to be considerably lower, some success has also been achieved with the intercalation of alkaline earth dopants, such as Ca, Sr, and Ba [27, 28, 29], where two electrons per metal atom M are transferred to the Cgo molecules for low concentrations of metal atoms, and less than two electrons per alkaline earth ion for high metal atom concentrations. Since the alkaline earth ions are smaller than the corresponding alkali metals in the same row of the periodic table, the crystal structures formed with alkaline earth doping are often different from those for the alkali metal dopants. Except for the alkali metal and alkaline earth intercalation compounds, few intercalation compounds have been investigated for their physical properties. 

Raman spectra have also been reported on ropes of SWCNTs doped with the alkali metals K and Rb and with the halogen Br2 [30]. It is found that the doping of CNTs with alkali metals and halogens yield Raman spectra that show spectral shifts of the modes near 1580 cm" associated with charge transfer. Upshifts in the mode frequencies are observed and are associated with the donation of electrons from the CNTs to the halogens in the case of acceptors, and downshifts are observed for electron charge transfer to the CNT from the alkali metal donors. These frequency shifts of the CNT Raman-active modes can in principle be u.sed to characterise the CNT-based intercalation compound for the amount of intercalate uptake that has occurred on the CNT wall. 

Lerf A (2004) Different modes and consequences of electron transfer in intercalation compounds. J Phys Chem Sol 65 553-563... 

X-Ray studies confirm that platinum crystallites exist on carbon supports at least down to a metal content of about 0.03% (2). On the other hand, it has been claimed that nickel crystallites do not exist in nickel/carbon catalysts (50). This requires verification, but it does draw attention to the fact that carbon is not inert toward many metals which can form carbides or intercalation compounds with graphite. In general, it is only with the noble group VIII metals that one can feel reasonably confident that a substantial amount of the metal will be retained on the carbon surface in its elemental form. Judging from Moss s (35) electron micrographs of a reduced 5% platinum charcoal catalyst, the platinum crystallites appear to be at least as finely dispersed on charcoal as on silica or alumina, or possibly more so, but both platinum and palladium (51) supported on carbon appear to be very sensitive to sintering. 

In alkali metal intercalation compounds, the guest is ionised in the host, donating its outer s electron to the host s electronic energy levels. Thus there are two aspects to consider, the sites where the ion resides, and the energy levels or bands that the electron occupies. Guests such as water that remain neutral will only be discussed in the section on cointercalation. In some hosts, notably graphite, some guests accept electrons from the... 

One complication in applying these models to intercalation compounds is in treating the dissociation of the intercalated atom into ions and electrons. The chemical potential can be written as a sum of contributions from ions and electrons, according to... 

The electrons, if they are separated from the ions, will also contribute to the entropy, and one might naively expect an expression similar to Eqn (7.8). Then the chemical potential for an atom would be the sum of two terms like Eqn (7.10), one from ions and one from electrons, and so the entropy term would be doubled. This is not so, however, in metallic intercalation compounds. In metals, the entropy of electrons is small. Electrons added by intercalation do not have a choice of all the empty states in a band, but only those within kT of the Fermi energy. If the Fermi energy is expressed as a temperature Tp and is measured from the bottom of the band, the change in entropy with the number n of electrons, dS/dn, is of order kTfTj (Kittel, 1971), not of order k like Eqn (7.8) for... 

The potassium donates an electron to the graphite (forming and the conductivity of the graphite increases. Graphite electron-acceptor intercalation compounds have also been made with NO3T CrOs, Br2, FeCls, and ASF5. Some of these compounds have electrical conductivity approaching that of aluminum (see Chapter 6). 

Titanium disulfide has a Cdl2 structure (see Chapter 1). The solid is golden-yellow and has a high electrical conductivity along the titanium layers. Forming intercalation compounds with electron donors can increase the conductivity of titanium disulfide, the best example being with lithium, LLTiS2. This compound is synthesized in the cathode... 

The fust intercalation compound was made in 1841. This contained sulfate, an electron acceptor. Since then, many other electron acceptor intercalation compounds have been made with, for example, NO3T CrOs, Br2, FeCls, and AsFs. In these compounds, the graphite layers donate electrons to the inserted molecules or ions, thus producing a... 

Both of the potassium and polybromide intercalation compounds are good conductors of electricity. In the potassium inlercalam. the electrons in the conduction band can carry the current directly, as in a metal. In the compounds of graphite with polybromide. holes in the valence band conduct by the mechanism discussed previously for semiconductors (Chapter 7). 

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