Visualization of topotactic structural transformations of zeolites using 3D electron diffraction

Knowing the structures of intermediates in phase transformations is crucial for elucidating the formation mechanisms and tailoring the properties of many industrially important materials, ranging from ceramics1 alloys/metal oxides2-5and battery cathodes6-10 to nanoporous materials11-19and pharmaceuticals.20-22 However, the intermediates of many these materials consist of submicrometre- or nanometre-sized crystals and their structures often exhibit large similarity and/or contain disorder.6,7,12-16 Particularly, intermediate structures resulting from topotactic reactions and transformations can even share the same unit cell and include partially occupied atoms.6,7,10,12-14 These pose challenges for conventional powder X-ray diffraction in distinguishing detailed structural variations.22 Here we present the discovery of two pure-silica extra-large pore zeolites ECNU-45 and ECNU-46 and visualization of detailed structural changes of reaction intermediates during topotactic transformation from ECNU-45 to ECNU-46. We show that minor differences in complex isocellular zeolite structures with partially occupied and/or disordered atoms can be determined using three-dimensional electron diffraction (3D ED). Six topologically distinct tetrahedral Si sites were visualized to have varied occupancies in the intermediate structures, demonstrating where the topotactic reactions occurred directly. This further enables visualization of the gradual atomic structure changes and pore architecture transformations of ECNU-45 (3D, 24×10×10-ring) into ECNU-46 (1D, 24-ring). Our results open the way to elucidating complex solid-state reaction and phase transformation mechanisms of polycrystalline materials at atomic-scale for new materials engineering.