Layered Solids and Intercalates

Finally, the layered solids and intercalates of inorganic clay minerals and graphite as well as the zeolites as porous aiuminosiHcates have to be quoted here. " ... 

Many layered solids form intercalation compounds, where a neutral molecule is inserted between weakly bonded layers. For example when potassium vapour reacts with graphite above the melting temperature of potassium (337 K), it forms a golden compound KCs in which the potassium ions sit between the graphite layers, and the inter-layer spacing is increased by 200 pm (Figure 3.16). Addition of a small amount of KO2 to the molten potassium results in the formation of a double layer of potassium atoms between the graphite layers and a formula close to KC4. 

Intercalation in layered solids is a long-established phenomenon. It has been suggested [ 1 ] that the first example, dating from over two thousand years ago, involved intercalation in kaolinite (an aluminosilicate clay) and explains the secret behind the production of fine Chinese porcelain, hi modern times, many thousands of papers have been devoted to intercalation chemistry in clays, graphite and other materials. 

Layered structures are also found for many oxides and sulfides of transition metals. They can be intercalated with alkali metals (Li, Na, K) to give superconducting solids and conducting solids that are useful for solid state battery materials. 

Five aspects of the preparation of solids can be distinguished (i) preparation of a series of compounds in order to investigate a specific property, as exemplified by a series of perovskite oxides to examine their electrical properties or by a series of spinel ferrites to screen their magnetic properties (ii) preparation of unknown members of a structurally related class of solids to extend (or extrapolate) structure-property relations, as exemplified by the synthesis of layered chalcogenides and their intercalates or derivatives of TTF-TCNQ to study their superconductivity (iii) synthesis of a new class of compounds (e.g. sialons, (Si, Al)3(0, N)4, or doped polyacetylenes), with novel structural properties (iv) preparation of known solids of prescribed specifications (crystallinity, shape, purity, etc.) as in the case of crystals of Si, III-V compounds and... 

Intercalation of polyaniline between MoS2 layers produced nanoscale molecular composites with unusual charge transport properties [119]. Recent advances in the preparation, characterization, and utilization of conducting polymers intercalated into layered solids were surveyed [120],... 

The nature of the surface promoter species has been debated for many years. As MoS2 crystals exist as layered structures, two models evolved. One proposed that the promoter was intercalated deep within the bulk of the MoS2 layers (51) and another proposed that bulk intercalation was not thermodynamically stable and a surface-intercalated structure was more likely (52) (see Fig. 16d). Both proposals related the promotion to crystal surface reconstruction and solid-state chemistry and are valid only for multilayered structures. 

The swelling of clays in water results from the extra hydration of the interlamellar cations (Fig. 77). This is the best known example of the important phenomenon of intercalation, which is simply the insertion of guest species into an accommodating host, usually, but not exclusively, a layered solid. The degree of swelling, however, is governed by the nature of the interlamellar cation and the sorption isotherm often exhibits steps, as so often occurs with clathrates. 

The layered arrangement makes these materials very interesting from the point of view of host-guest behaviour because ionic or molecular guest species may be inserted between one layer and another causing the layers to expand or swell. Guest intercalation is generally reversible, and it is an important characteristic of layered solids that, rather like zeolites, they can retain their layered host structure... 

Layered solids such as graphite are interesting in separation and sorption applications and can be doped to give interesting materials properties as in Li ion batteries. Their intercalation behaviour is best described by the Daumas-Herold model. 

Alkali metals and bromine react with graphite to form solids known as intercalation compounds, where the foreign atoms are inserted between the intact graphite layers. Many other layered solids, for example dichalcogenides such as TaS2, which have structures similar to Cdl2 will also for intercalation compounds. The inserted species may be alkali metals, or electron donor molecules such as amines or organometallic compounds. 

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