Electron crystallography as a complement to X-ray powder diffraction techniques

Abstract

X-ray powder diffraction / HRTEM / SAED / PED structure solution Abstract. Electron microscopy techniques yield informa- tion for crystal structure analysis that is remarkably com- plementary to that obtained from X-ray powder diffraction data. Structures of polycrystalline materials that resist so- lution by either method alone can sometimes be solved by combining the two. For example, the intensities extracted from an X-ray powder diffraction pattern are kinematical and can be interpreted easily, while those obtained from a typical selected area electron diffraction (SAED) or preces- sion electron diffraction (PED) pattern are at least partially dynamical and therefore more difficult to use directly. On the other hand, many reflections in a powder diffraction pattern overlap and only the sum of their intensities can be measured, while those in an electron diffraction pattern are from a single crystal and therefore well separated in space. Although the intensities obtained from either SAED or PED data are less reliable than those obtained with X- rays, they can be used to advantage to improve the initial partitioning of the intensities of overlapping reflections. However, it is the partial crystallographic phase informa- tion that can be extracted either from high-resolution trans- mission electron microscopy (HRTEM) images or from PED data that has proven to be particularly useful in com- bination with high-resolution X-ray powder diffraction data. The dual-space (reciprocal and real space) structure determination programs Focus and Superflip have been shown to be especially useful for combining the two dif- ferent types of data.