Successive phase transitions in perovskite crystals ABX3 due to condensations of Γ15 and R25-M3 modes at the center and R-M corners of Brillouine zone are described and the following three characteristics are discussed. 1) An alienating property between the condensation of Γ15 mode and those of the zone boundary R25-M3 modes is noticed to exist except SrTiO3. Such property is interpreted as due to coupling constant between the B-ion and X6-octahedron in Γ15 soft mode. 2) The condition for successive and nonsuccessive phase transition observed in these crystals is characterized by a ratio √‹u2›/ΔZ2, where ‹u2› is the mean square ionic displacement due to thermal vibration and ΔZ is the ionic shift in condensed phase. 3) The extremly anisotropic diffuse scatterings found in X-ray and electron diffraction photgraphs can be regarded as a premonitory property indicating the direction of ionic shifts which appear immediatly after the soft mode is condensed.
The article briefly describes the recent progress on the molecular structure and the function of nucleotide coenzymes, especially nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), which are relatively small molecules but play a very important role in the enzymatic reactions. The molecular conformation of NAD elucidated from several solution studies by NMR, fluorescence etc. do not fully correspond to that found in enzyme-NAD complex and even in NAD+·Li+ crystal, whereas the crystal structure of FAD is still unknown at the moment, and most plausible molecular conformation of FAD is proposed by conformational analysis based on the structural data of riboflavin-adenine molecular complex.
X-ray diffraction images of plate-like defects, twin and misorientation boundaries, were studied by means of computer simulation. The diffraction theory for the simulation is outlined. The simulated patterns are obtained on a line printer while the numerical output is also available. An observed topograph of quartz dauphiné twin was compared with the pattern simulated with a reasonable set of parameters. The correspondence was reasonably satisfactory.