Intercalation alkali atoms

Protons are not the sole species that can be incorporated into the lattices of different host materials. At the beginning of the 1960s, Boris N. Kabanov showed that during cathodic polarization of different metals in alkaline solutions, intercalation of atoms of the corresponding alkali metal is possible. As a result of such an electrochemical intercalation, either homogeneous alloys are formed (solid solutions) or heterogeneous polyphase systems, or even intermetallic compounds, are formed. 

The Cfto molecules form a cubic lattice in which the molecules are weakly bound by Van der Waals forces, and most of the molecular properties survive in the crystal. The molecular energy levels broaden into narrow bands, the separation between the HOMO and LUMO levels being of the order of 1 eV (Fig. 1). During the intercalation of alkali atoms into the octahedral and tetrahedral sites of the cubic lattice, the electrons are donated to the triply degenerate (t, )... 

The approximately constant expansion of the graphite lattice, which is practically independent of the size of the intercalated metal atom, appears at first sight unusual for, in the ammonia-free compounds, the distance between the layer planes increases, as expected, with increasing size of the alkali metal atom from potassium to cesium. The constancy of the expansion for the ammoniates is perhaps attributable to the effect of the positions of the ammonia molecules in the lattice in determining the increase in the interplanar distance. If this were so, metal atoms or ions could perhaps find sufficient room in holes in the ammonia lattice. In support of this view it may be added that the expansion becomes greater if, in place of ammonia, a layer of amine such as methylamine or ethylamine is intercalated. 

The first experiments were carried out on films of solid CeO) 100 to 1000 A thick, exposed to alkali metal vapors.[Ha91a ] It was observed that the conductivity increased by more than seven orders of magnitude, to 500 (fl cm) . This is still a much lower conductivity than any metallic element, but on the order of such organic systems as doped polyacetylene. Haddon et al. made the plausible suggestion, subsequently proved, that the smaller alkali ions were intercalated into the voids between the much larger fullerenes, and donated their charge to the unoccupied fullerene molecular orbital. The ratio of alkali atoms to fuUerenes in the (super) conducting phase was not then known, nor was it known whether the structure was based on the... 

Doped fullerites are called fullerides. The doping process proceeds by intercalating electroactive atoms or molecules into the crystal lattice in a very similar way as it is well known for the conducting polymers. Sofar only strong donors like alkali metals or earth alkali metals were found to dope the fullerites if the latter are exposed to the vapor of the metal. Doping induced conductivity has been observed for various metals and various fullerenes but metallic behavior and superconductivity was found sofar only for Ceo- Table 1 compiles a selected number of doped systems together with some of their transport properties. 

Another property of transition metal dichalcogenides is an intercalation effect. Most of the atoms in the periodic table and organic molecules which are Lewis bases can be intercalated in the van der Waals gap sites, forming intercalation compounds(intercalates). One of the most striking features is a transport phenomenon. Alkali metal intercalates of IV] - and VIb-MX2, for example, NaxMoS2 or LixZrSe2 become metallic and show a superconductivity at low temperatures(10,11). A charge transfer occurs between the alkali atom and the mother crystal. 

The alkali atom behaves as a donor intercalant and transfers a 2s-electron into the vacant band of the mother crystal. The LixZrSe2 system was precisely investigated by Berthier et al.(12). They claimed the occurrence of a non-metal to a metal transition at x=0.4 in LixZrSe2 through X-ray, electrochemical, conductivity, NMR and EPR measurements. 

Figure 5. It is likely that the intercalated alkali cations are surrounded by ethereal oxygen atoms like the cations in crown ethers, the iodine and thiocyanate anions being associated with the cations. It is interesting to note that the organic derivatives of y-zirconium phosphate synthesized in this study can form the complexes only with the alkali salts having soft base anions such as I and SCN , but not with the salts having hard base anions such as Br and N03 .

Graphite reacts with alkali metals, for example potassium, to form compounds which are non-stoichiometric but which all have limiting compositions (for example K C) in these, the alkaU metal atoms are intercalated between the layers of carbon atoms. In the preparation of fluorine by electrolysis of a molten fluoride with graphite electrodes the solid compound (CF) polycarbon fluoride is formed, with fluorine on each carbon atom, causing puckering of the rings. 

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. 

One of the most widely explored systems is derived from the interpolation of Li between the TiS2 layers in varying amounts to form nonstoichiometric phases with a general formula LivTiS2. Because the bonding between the layers is weak, this process is easily reversible. The open nature of the structure allows the Li atoms to move readily in and out of the crystals, and these compounds can act as convenient alkali metal reservoirs in batteries and other devices. A battery using lithium intercalated into TiS2 as the cathode was initially developed some 30 years ago. 

Omloo and Jellinek7 have described the synthesis and characterization of intercalation compounds of alkali metals with the group V layered transition metal dichalcogenides. Typically, these types of intercalation complexes are sensitive to moisture and must be handled in dry argon or nitrogen atmospheres. The alkali metal atoms occupy either octahedral or trigonal prismatic holes between X-M—X slabs. There are two principal means by which these compounds may be prepared. 

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