Advanced techniques to define intercalation processes

Intercalation is the inclusion or reversible insertion of a guest chemical species (atom, ion, molecule) in a virtually unchanged host-crystal structure. Any type of layer-structured material may give rise to intercalated compounds, the guest species being artificially inserted or naturally included between the host sheets without loss of their planarity. Layer silicates, in particular, may be considered intercalated structures where interlayer guest species and complexes are inserted between the silicate layers. The most common guest species is H2O, which is generally present under natural conditions in intercalated layer silicates such as smectites, vermiculite and halloysite. Past research focused attention on the swelling/shrinking behaviour of intercalated compounds with respect to H2O, and also on the non-stoichiometric, heterogeneous complexes formed from organic liquids such as ethylene glycol and glycerol. The unique combination of layer-silicate features (small crystal size, large surface area) and the small concentrations required to effect a change in the matrix, both coupled with the advanced characterization techniques available, have generated much interest. This interest extends to the special field of nanocomposites, and of graphene, which is also an intercalated layered structure. In general, any guest material inserted into an interlayer space causes a modification in the structure, with spacing-size changes in a particular crystallographic direction (d value). First, a brief introduction on conventional and synchroton-based X-ray techniques used to define crystal size and thickness is given. Then, the peak-broadening approach by conventional X-ray diffraction (XRD) techniques, such as the Scherrer method is presented. Further on, the crystallinity measurements and the Bertaut-Warren-Averbach (BWA) method used in the MudMaster program are described. A short summary is presented of the grazing-incidence diffraction (GIXRD) technique. Finally, additional and complementary information from X-ray absorption spectrometry (XAS), such as short-range order, and detailed local information on atomic positions by angle-resolved X-ray absorption near-edge stucture (AXANES), polarized extended X-ray absorption fine structure (P-EXAFS), and near-edge extended absorption fine structure (NEXAFS) spectroscopies are analysed and discussed. Examples of the applications of these methods to clay minerals, micas and graphene are given.