Insight Into the Defects of Cage-Type Silica Mesoporous Crystals with Fd(3)over-barm Symmetry: TEM Observations and a New Proposal of "Polyhedron Packing" for the Crystals

Silica mesoporous crystals were synthesized by using a gemini cationic surfactant (C18-3-1) as the directing agent, carboxyethylsilanetriol sodium salt as the co-structure directing agent (CSDA), and varying amounts of HCl. By using transmission electron microscopy (TEM) we observed 1) a structural change from the close-packed structures of spherical micelles-face-entered cubic (Fm (3) over barm) and hexagonal close-packed (P6(3)/mmc)-to Fd (3) over barm structures with an increase of HCl and 2) a few structural defects in the crystals with Fd (3) over barm symmetry. The structure of a crystal with Fd (3) over barm symmetry is described as one of the tetrahedrally close-packed (tcp) structures consisting of 5(12) and 5(12)6(4) polyhedra. The observed TEM images of the structural defects were explained well through use of simulated TEM images by introducing new 13-15 polyhedra comprising 5(12)6(2), 5(12)6(3), 4(1)5(10)6(2), 4(2)5(8)6(5), and 4(1)5(10)6(4), which have been observed in bubbles by Matzke. The mesostructural changes and defect formation are discussed in terms of the hardness of micelles composed of surfactant/CSDA/silica species that have formed through a change of the interaction between the surfactant and CSDA, which causes the micelles to change from a regime of close-packing to one of minimum-area packing.