Application of combining X-ray diffraction and electron crystallography for determination of complex inorganic crystal structure

Abstract

Inorganic, organic, and biological materials have specific natural properties which mostly depend on their atomic structures. The properties of novel materials can be predicted based solely on knowing the structure fully. Thus, structure determination plays a very important role in chemistry, physics, and materials science. X-ray crystallography, including single-crystal X-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD), remains an important technique for studying structures. However, SCXRD can only be applied to high-quality large single crystals without disorders/defects, whereas PXRD provides only one-dimensional information and reflections with the similar d-values will overlap, which makes it difficult to determine the unit-cell parameters, space groups, and accurate intensities. Another important technique for structural determination is electron crystallography (EC). As the electron is the probe, EC alone can be used for those crystals which are too small to be studied by SCXRD or too complex to be studied by PXRD. Electrons interact much more strongly with matter than X-rays; therefore, both electron diffractions (ED) patterns and high-resolution transmission electron microscopy (HRTEM) images can be obtained from nano-sized crystals. Although electron crystallography started later than X-ray crystallography, it has become a very important technique for structural analysis after several decades of development. Especially, three dimensional (3D) ED techniques have been developed, automated electron diffraction tomography (ADT) and rotation electron diffraction (RED), which allow for automated data collection without requiring considerable expertise on the operation of electron microscopes. In addition, the intensities of 3D ED data can be extracted and used for structure determination using specialized software developed for SCXRD. However, the strong interactions between electrons and materials also result in dynamic effects and beam damage. Although the dynamic effects in 3D electron diffraction techniques (ADT and RED) can be significantly reduced, some structures still pose problems for obtaining an initial model due to beam damage. Therefore, EC and X-ray crystallography have significant limitations. For many complicated crystals, a single technique is insufficient to solve the crystal structure and different techniques that supply complementary structural information must be used to obtain a complete structural determination. Herein, the application of X-ray crystallography combined with EC for the analysis of complex inorganic crystal structures will be introduced, covering issues associated with peak overlap, impurities, pseudo-symmetry and twinning, disordered frameworks, location guests, and aperiodic structures.