Structural Analyses of Intergrowth and Stacking Fault in Cage-Type Mesoporous Crystals

A mixture of Gemini cationic surfactant, costructure directing agent (CSDA) with carboxylate group, and silica oligomer leads to the formation of cage-type mesoporous crystals, and the system changes the structure from Fm3̅m (or P63/mmc) type to Fd3̅m type with an increasing amount of additive (HCl). Here we have investigated the Fm3̅m−Fd3̅m type intergrowth and new type stacking fault observed in the intermediate phase using transmission electron microscopy (TEM) and proposed a layer-by-layer growth mechanism. In order to describe the structures, four types of polyhedra for the tetrahedrally close-packed structures are introduced. In the intergrowth, the structure changes the packing of spherical micelles between the Fm3̅m (or P63/mmc) and Fd3̅m structures depending on the pH. The results can be explained by the change between a hard sphere characteristic (perfect sphere) in the Fm3̅m structure and a soft sphere characteristic (polyhedron) in the Fd3̅m structure. The stacking fault in the Fd3̅m structure can be explained by new arrangement of two polyhedra, the 14- and 15-hedron.