Guest intercalation compounds

Insertion (intercalation) compounds. Insertion compounds are defined as products of a reversible reaction of suitable crystalline host materials with guest molecules (ions). Guests are introduced into the host lattice, whose structure is virtually intact except for a possible increase of some lattice constants. This reaction is called topotactic. A special case of topotactic insertion is reaction with host crystals possessing stacked layered structure. In this case, we speak about intercalation (from the Latin verb intercalare, used originally for inserting an extra month, mensis intercalarius, into the calendar). 

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... 

Fig. 7.2 Classification of intercalation compounds (a) host of chains weakly bonded together (b) three-dimensional host with one-dimensional lattice of sites for guest ions (c) layered host (two-dimensional host and two-dimensional lattice of sites) (d) three-dimensional host with three-dimensional lattice of sites.

Figure 9.13 Classical and Daumas-Herold model of staging in intercalate compounds. The stage number represents the ratio of host to guest layers.

In structure determination from X-ray diffraction data, it sometimes happens that, on the Fourier maps, parts of the coming out structure are unclear. Fuzzy electron density maps may present problems in determining even the approximate positions of the respective fragments of the structure being analyzed. For example, the layered structure of the inclusion (intercalation) compound formed by Ni(NCS)2 (4-methylpyridine)4 (host) and methylcellosolve (guest) [1], The guest molecules are (Fig. 11.1) located on twofold crystal axes of unit cell symmetry and are orientationally disordered as shown in the picture. 

In BLH-NhaCo()2, the guest species in the galleries are highly disordered. MEM is quite effective for detailed structure analysis of such an intercalation compound. In MEM-based whole-pattern fitting (MPF), crystal structures are expressed not by structure parameters such as fractional coordinates and atomic displacement parameters but by electron densities in the unit cell. Therefore, MPF allows us to represent the disordered atomic configuration in a more appropriate way than conventional Rietveld analysis adopting a split-atom model. The... 

Fig. 8. Schematic diagram of staging in intercalation compounds. Guest molecules are represented by circles in between the layers (shown by lines).

TABLE 3. Examples of hosts and guests in intercalation compounds... 

Figure 7-7 The various stages of graphite intercalation compounds. The letters A and B refer to the stacking pattern of carbon layers, and the two carbon layers flanking a guest layer are always equivalent, that is, have their carbon atoms superposed.

Figure 6 Schematic representation of staging in intercalation compounds. The lower part represents the isolated island or domain model. The continuous line represents the host layer, the dashed line a guest layer...

Graphite intercalation compounds have received widespread interest for two main reasons. First, intercalation results in compounds that have metallic conductivity. The electrical conductivity can approach that of copper metal and is highly anisotropic ratios of the conductivity in-plane to that along the c axis can be six orders of magiutude at room temperature. Secondly, graphite is a unique host lattice in that intercalation compounds can be formed with either electron donors or electron acceptors as guest species. 

Intercalation compounds with electron acceptors form a large class of compoimds with several distinct types of guest, including anions of strong acids, halogens and halides,... 

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