Structure determination of a disordered, complex zeolite by combining rotation electron diffraction and HRTEM

Abstract

Zeolites are porous materials with important industrial applications [1]. Their structures are complex, and sometimes disordered which makes their structure characterization difficult using conventional methods. Electron diffraction (ED) combined with high resolution transmission electron microscopy (HRTEM) is a very powerful method for determination of complex or disordered structures from nano‐and submicron‐sized crystals. In this work, we used Rotation Electron Diffraction (RED) [2] combined with HRTEM imaging and powder X‐ray diffraction (PXRD) to solve and characterize the structure of the complex germanosilicate IM‐18 [3]. The RED method combines discrete goniometer tilt steps (2.0‐3.0°) with fine beam tilt steps (0.05‐0.20°) to collect 3D ED data from a single particle. More than 1000 ED frames can be collected in less than one hour. Moreover, both sharp spots and diffuse streaks indicating the disorder could be seen from the 3D reciprocal lattice reconstructed from the RED data. IM‐18 was first prepared more than 8 years ago, but its structure remained unsolved [3]. RED data of IM‐18 consisting of 649 ED frames were collected covering a tilt range of 119.46° with a tilt step of 0.20°. RED data were collected at 200 kV using the software RED – data collection on a JEOL JEM2100 TEM. The RED data processing software was used for unit cell determination, indexing, and intensity extraction. We have developed a method and a software, QFocus , for structure projection reconstruction from a through‐focus series of HRTEM images acquired with a constant step of defocus changes [4]. The defocus value and two‐fold astigmatism were then determined for each image in the series. A contrast transfer function (CTF) correction was performed on each image and a final image representing the projected potential of the specimen was reconstructed by averaging all CTF‐corrected images. Several through‐focus series of 12 HRTEM images with a defocus step of 85.3Å were taken from IM‐18 along the b ‐axis, and the structure projections were reconstructed using QFocus . The unit cell parameters of IM‐18 were obtained from the 3D reciprocal lattice reconstructed from the RED data (Fig. 1), and refined against the PXRD data. The crystal of IM‐18 is monoclinic ( P 2/ m ) with a = 10.336 Å, b = 14.984 Å, c = 17.734 Å, β = 106.9° (Fig. 2A). The structure was solved from the RED intensities by using direct methods and further confirmed by using HRTEM images (Fig. 2B) and PXRD data. IM‐18 contains three‐dimensional intersecting channels defined by 8, 10, 8 vertex‐sharing (Ge, Si)O 4 tetrahedra along a‐ , b‐ and c ‐axis, respectively. Disorder was determined from the reconstructed HRTEM images (Fig. 2C). We have shown here that the RED method can help to solve the structures from diffraction intensities and give the information about the disorder in the materials. The structure projection reconstruction method is important for beam sensitive materials, because it allows fast data collection without the need of manually optimizing the defocus. The contrast of the reconstructed HRTEM image is greatly improved and the image can be directly interpreted in terms of structure projection.