Neutron powder diffraction is a powerful technique to clarify the relationship between the crystal structures and the properties of functional materials. Since neutron atomic scattering factors do not depend on atomic numbers monotonically, neutron diffraction gives more reliable structural information than X-ray to detect light elements among heavy ones. There are more than 60 neutron powder diffractometers (NPD's) in the world. In Japan, the angle dispersive diffractometry is adopted in NPD's at JRR-3 while the time of flight diffractometry is adopted in NPD's at KENS and J-PARC. This article was written to introduce the two diffractometries and their NPD's.
The cation-cation repulsions across the shared edges of polyhedra in garnet structure are the most prominent between tetrahedral and dodecahedral cations, and this effect has a significant influence on cation distribution and symmetry change in garnets. Moreover, the shared edge of SiO6 octahedron in stishovite (SiO2), with rutile structure, is more compressible than the unshared edges; this can be explained by Si-Si repulsion across the shared edge. Thus, the cation-cation repulsions across the shared edges of polyhedra cannot be ignored as a significant factor governing the stability of compounds with edge-sharing structures.
Bloch wave degeneracies, which can be observed in high-energy electron diffraction, are summarized. Critical voltages of non-systematic reflections for 4 beam and 5 beam cases, and zone axis critical voltage effect are discussed. Bloch wave degeneracy observed in non-symmorphic space group is explained. Pseudo-critical voltage effect observed in non-centrosymmetric crystal is also discussed. The experimental measurement of the degeneracy is mentioned briefly. Examples of application of the critical voltage effect for estimation of accurate structure factors are discussed.
The statistical dynamical theory proposed by Kato is expected to bridge the gap between dynamical and kinematical theories and interpret the diffractions from perfect to imperfect crystals. The theory considers the optical coherence related to the statistical nature of crystal imperfection. Although the theory is still in a developing stage, it has been applied to characterize crystal imperfection. In the present paper, the basic idea of the theory is presented. Then the results of experimental tests and the applications to crystal characterization are shown citing a few examples.
we proposed a new processing technique for protein crystals, pulsed UV laser soft ablation (PULSA) . Several kinds of protein crystals were successfully processed by laser irradiation at a wavelength of 193 nm. Unchanged resolution limit between pre- and post-processings in X-ray diffraction measurements showed PULSA had little influence on the crystallinity of the non-irradiated section. This technique will be a powerful tool for processing fragile biological-macromolecule crystals and will accelerate protein structural analysis.
GaN (Gallium Nitride) is an important material in optoelectronic devices for blue light-emitting diodes and lasers. Large-size single crystals of GaN are strongly desired, which can be used as substrates for epitaxial growth. We made in situ X-ray diffraction experiments using a multi-anvil high-pressure apparatus at the SPring-8 to confirm its decomposition and melting behavior under high pressures and temperatures. Congruent melting of GaN occurred around 2220 °C at pressures above 6.0 GPa, and decreasing the temperature allowed the GaN melt to crystallize to the original structure. This result leads to a new synthesis method of high quality single crystals of GaN by means of slow cooling of its stoichiometric melt under high pressure. Single crystal of GaN with a diameter of 100 μm has been obtained successfully that shows a sharp X-ray rocking curve smaller than 30 seconds.