Phasing via electron and neutron diffraction data

Abstract

Among the statistics freely available on the webpage of the Cambridge Structural Database, there is a detail of interest for this chapter: of the 596 910 crystal structures deposited up to 1 January 2012, only 1534 were solved by neutron data (see Table 1.11). No information is provided on the number of structures solved by electron data because it is negligible (organic samples are soon damaged by the electron beams). A statistical search of the Inorganic Crystal Structure Database (ICSD, Ver. 2012–1, about 150 000 entries; by courtesy of Thomas Weirich) on structures that have been solved by means of electron diffraction, eventually in combination with other techniques, indicates a total of about 0.7%. In spite of limited impact on the databases, electron and neutron diffraction play a fundamental role in materials science and in crystallography. The main reason is that they provide alternative techniques to X-rays. Let us first consider electron diffraction (ED) techniques. The study of crystalline samples at the nanometer scale is mandatory for many industrial applications; indeed, physical properties depend on the crystal structure. Unfortunately it is not unusual for compounds to only exist in the nanocrystalline state; then, traditional X-ray diffraction techniques for atomic structure determination cannot be applied, because of the weak interactions between X-rays and matter. As a consequence, such structures remain unknown, in spite of their technological importance. This limits the contribution of X-ray crystallography to nanoscience, a growing scientific area, crucial to many fields, from semiconductors to pharmaceuticals and proteins. The result is a lack of knowledge on the underlying structure–property relationships, which often retards further research and development. Structure analysis by electron diffraction began as early as the 1930s (in particular, by Rigamonti, in 1936), but the interest of the crystallographic community in such a technique soon faded, mostly because electron diffraction intensities are not routinely transferable into kinematical |F|2. In spite of this limitation, the technique has been used for investigating the structure of many inorganic, organic, and metallo-organic crystals, biological structures, and various minerals, especially layer silicates.