Layer lattice compounds intercalation

Many layer-lattice compounds can intercalate additional metal atoms of the same element as comprised in the original structure (e.g. niobium in niobium diselenide), but molybdenum disulphide will not do so. The behaviour may be determined by the availability of electrons suitably oriented to form bonds with the additional metal atoms, although it seems unlikely that this single factor applies to all intercalation effects. 

A new class of compounds in which hydrogen bonding is important is that of the molecular intercalates. These are a special type of inclusion compound, formed by the insertion of molecules into empty sites between layers of a layered structure. All intercalation reactions are characterized by an expansion of the crystal lattice along the c direction perpendicular to the layers to an extent that may be correlated... 

It is possible to insert additional atoms or molecules into the inter-lamellar gap of many layer-lattice materials, including molybdenum disulphide, creating what are called intercalation compounds. The intercalated substances may be alkali or alkalyne-earth metals (sodium, potassium, rubidium, caesium, calcium, strontium), salts or organic bases such as ethylene diamine or pyridine . 

Despite these restrictions, many families of compounds undergo intercalation reactions including chain structures, layered lattices and three-dimensional connected frameworks with tunnels or channels. 

At room temperature, the intercalated layers of a stage two nitric acid compound are disordered in a liquid-like state modulated by the graphite lattice. The intercalated nitric acid molecules are neither organized in double layers, as suggested earlier by Rudorff , nor oriented parallel to the graphite c axis. Instead, the molecule planes are tilted by about 30° to the c axis, as deduced from neutron spectroscopy and from X-ray data". 

Ion conductors are usually compounds in which the atoms are bonded together with an ionic bond. Textbooks on ion conduction do not spend much time on bonding and for ionically conducting solids the classical electrostatic Coulomb interaction suffices. However, there also exist covalent layered lattices, which admit intercalation of ions between the covalent layers, and these lattices can conduct electrons as well as interstitial ions. Good ion conductors have ions that in their bond with the other atoms in the lattice have no strong preference for one particular coordination number, but balance between two values. This lowers the activation barrier of the ion path through the lattice and results in a higher ionic conduction. 

These intercalates are not randomly distributed between the graphene layers but have well-defined positions of maximum stability related to their interactions and the electronic state of the graphene layers. The penetration between the graphene layers of these intercalates results in the expansion of the graphite lattice (swelling) with the formation of the so-called intercalation compounds (Tanaike and Inagaki, 1999). These intercalation compounds are not thermally stable and react easily with water vapor. 

Of special interest to intercalation studies are complex non-stoichiometric systems, such as the so-called misfit layer chalcogenides that were first synthesized in the 1960s [45]. Typically, the misfit compounds present an asymmetry along the c-axis, evidencing an inclination of the unit cell in this direction, due to lattice mismatch in, say, the -axis therefore these solids prefer to fold and/or adopt a hollow-fiber structure, crystallizing in either platelet form or as hollow whiskers. One of the first studied examples of such a misfit compound has been the kaolinite mineral. 

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). 

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.

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