Structure determination from laboratory powder diffraction data can be considered routine today as emphasized by the significant increase in the number of published structures, both organic and/or inorganic compounds solved in this way. In the past about 10 years, many significant developments in instrumentation, such as computer technology and powder diffraction methodology, allow determination of crystal structures, the size and complexity of which are only limited by data quality and computing power. New detector technologies (mostly strip detectors), faster PCs and powerful methods of structure solution (especially the direct space and the charge flipping methods) contributed most to an ever increasing success rate. Also monochromater for incident beam (like CuKα1 or MoKα1) is not always required for structure determination.
Today, laboratory X-ray diffractometers are seeing increasingly wide use in the ab initio crystal structure analysis of organic powder samples. This is because optics and optical devices have been improved, making it possible to obtain precise integrated intensities of reflections in high 2-theta ranges. Another reason is that one can use direct-space methods, which do not require “high-resolution diffraction data”, much more easily than before. Described here are some key points to remember when performig ab initio crystal structure analysis using powder diffraction data from organic compounds.
The interplay between magnetism and electricity in matter has become a central issue of condensed-matter physics. Here I focus on the ferroelectricity induced by magnetic order mostly in frustrated magnets, which is nowadays referred to as magneto-electric (ME) multiferroic. Some distinct types of microscopic origins relevant to the spin-driven ferroelectricity are discussed. It is also shown that the frustration based spin-driven ferroelectrics can exhibit nonlinear and giant ME responses of phase transition type.
Local structural order in a metallic glass was investigated by Angstrom beam electron diffraction using an aberration corrected scanning transmission electron microscope. The coherent electron probe with a diameter of 0.3 - 0.4 nm was successfully obtained by using Ronchigram method. The fine electron probe enables us to pick up Angstrom-scale local structure information which cannot be obtained by conventional diffraction methods and thereby provide a direct evidence of short- to medium-range order in metallic glasses with the help of ab-initio molecular dynamics simulation.
Recent technical developments in macromolecular X-ray crystallography have significantly improved the resolution limit of protein structures. However, numbers of high-resolution structures are still limited. In this study, the X-ray crystal structure of bovine H-protein, a component of the glycine cleavage system, was determined at 0.88 Å resolution. This is the first ultrahigh-resolution structure of an H-protein. The data were collected using synchrotron radiation. Because of limitations of the hardware, especially the dynamic range of the CCD detector, three data sets (high-, medium- and low-resolution data sets) were measured in order to obtain a complete set of data. To improve the quality of the merged data, the reference data set was optimized for merging and the merged data were assessed by comparing merging statistics and R factors against the final model and the number of visualized hydrogen atoms. In addition, the advantages of merging three data sets were evaluated. The omission of low-resolution reflections had an adverse effect on visualization of hydrogen atoms in hydrogen-omit maps. Visualization of hydrogen electron density is a good indicator for assessing the quality of high-resolution X-ray diffraction data.
In magnetic crystals belonging to chiral space groups, the crystallographic chirality affects the arrangement of spin magnetic moments. The crystallographic symmetry plays a role of a “mold” which pins down the spatial arrangement of the spins. Consequently, in this category of magnetic crystals it becomes possible to control such abstract quantities as the spin chirality or the phase of the spin’s wave function. Based on these ideas, we review our recent progress on chiral magnetic crystals.
Electron diffractive imaging by using selected area nano diffraction in an aberration-corrected transmission electron microscope has been developed. Atomistic structures in silicon <110> and <112> projections have been reconstructed by the method successfully. In the results, the dumbbell structures with the separations of 0.136 nm and 0.078 nm have been resolved clearly. Discrimination of different elements and visibility of light atom columns have been proved by reconstruction of magnesium and oxygen atom columns in a <110> projection of a MgO crystal.
The surface structures of SrTiO3 thin films homoepitaxially grown by pulsed laser deposition (PLD) were studied using low temperature scanning tunneling microscopy (STM). We suggest microscopic pictures of the near-surface stoichiometry of SrTiO3 thin films, depending on their thickness and the growth mode. For comparison, an atomically ordered surface of SrTiO3 single crystal that is robust under typical thin film growth condition is shown. We atomically visualize the initial growth of homoepitaxial SrTiO3 thin film on this well-defined substrate, indicating that the surface structure of the substrate can be transferred to the film surface. Atomic-scale understanding of the surface structure of the substrate and the initial process of the thin film growth will provide new approach to realize oxide heterointerfaces that are controlled truly on an atomic scale.