Layer Rigidity in Intercalation Compounds

Abstract

During the past several years there have been extensive experimental and theoretical advances in the understanding of the effect of microscopic layer rigidity on the macroscopic properties of pristine and intercalated layered solids. In this review we bring these experimental and theoretical developments together in a concise description which highlights the interplay between them. Since we and our students and colleagues were heavily involved in first defining and then elucidating the concepts of layer rigidity, we have drawn freely from our previous work to formulate the review presented here. Solin has classified layered solids into three qualitatively distinct subgroups1 on the basis of the rigidity of the layer units with respect to transverse distortions in which the constituent atoms are displaced in directions normal to the layer planes. According to this classification scheme, solids with monatomically thin host layers belong to Class I ; the members of which are graphite and boron nitride and their intercalation compounds. The layers of these solids are floppy with respect to transverse distortions. Class II contains solids, the layers of which are typically constructed from three planes of interconnected atoms. Materials such as the layer dichalcogenides, iron oxychloride and a number of metal halides belong to this group, whose layers are more rigid than those of the Class I solids. Class III solids, which include the layered alumino silicate clays and layered perovskites, are among the most rigid known in nature. These solids typically consist of five or more interconnected planes of atoms and are much more rigid than those in Classes I or II. The above classification can of course serve only as a rough guide. Thus it has recently been shown2 that some layer double hydroxides, though they exhibit Class II – like structures actually exhibit rigidities which span those of Class II and Class III. By definition, layered solids are those for which the intralayer interatomic forces binding the layers together are much stronger than the forces between layers. The resultant anisotropy gives rise to the phenomenon of intercalation whereby guest species can occupy the gallery spaces between the host layers. To first order the only perturbation of the host layers upon intercalation is their increased separation. In Class I and II materials, intercalation is mitigated by charge exchange between the guest species and the host layer.