Calcination of octadecasil: Fluoride removal and symmetry of the pure SiO2host

Pure silica octadecasil (AST) has been synthesized hydrothermally in a fluo ride medium using a new structure-directing agent (tert-butyltrimethylammon ium). F- is occluded in the material inside the small [4(6)] cage and is re moved completely upon calcination, in sharp contradiction with recent predi ctions based on computational studies. We propose chemically feasible pathw ays for the migration of F- out of the [4(6)] cage that are based on the kn own chemical reactivity of F- and SiO2 and on the presence of small cations formed by thermal degradation of the organic species. After calcination to remove the guest species, the structural integrity of the host is complete ly preserved, yielding a pure SiO2 framework devoid of connectivity defects . Its structure has been solved by direct methods and refined using synchro tron X-ray powder diffraction data. Despite the removal of guest species, t he framework is seen to adopt a tetragonal symmetry rather than the cubic m aximum topological symmetry. It is proposed that symmetry lowering in AST i s a consequence of the energetically unfavorable angles in the higher space group, in which 20% of the Si present four linear Si-O-Si angles. Si-29 MA S NMR spectroscopy and energy minimization calculations support this notion and the refined structure proposed. At 500 degrees C calcined octadecasil undergoes a reversible phase transition to pseudocubic symmetry. Finally, t he reported AIPO(4)-16 or octadecasil cubic forms are proposed to actually be disordered pseudocubic structures.