Based on the supposition that the non-diffusive solid-solid phase transition of the first order is also triggered by soft phonon, a study was made. Orientation relation between B1 and B2 structures was found to exist for alkali halides across the pressureinduced transition. Calculations show instability of TA  mode of B1 structure at high pressures. Energy barriers opposing the transition are calculated to be low enough at the equilibrium pressure, enabling transition at 0°K to take place at a reasonable value of pressure in agreement with experimental results of Laukhin et al. [J. Cryst. Soc. Jpn. 26, 157 (1984) ] .
Crystal structures of tantalum hydrides have been reviewed.The Ta-H system is characteristic of an interstitial alloy of hydrogen in bcc tantalum over a wide composition range. Hydrogen atoms in the interstitial sites become ordered at low temperatures and superstructures β1-Ta2H, β2-Ta2H, γ-Ta4H3 and δ-TaH are formed. These super-structures are closely related and transform to one another with hydrogen composition and temperature. The crystal structure of β1-Ta2H is discussed in detail on the basis of the recent neutron and X-ray diffraction work. [J. Cryst. Soc. Jpn. 26, 169 (1984) ] .
An investigation on the structure and its decomposition process of an Fe84B16 amorphous alloy by transmission electron microscopy is described. Under an optimum defocus condition, bcc-like microcrystalline domains with a size of about 12 Å were observed by the lattice imaging even in an as-quenched specimen. They were found to grow up to a size about 20 Å on annealing near 300°C. A formation of a halo electron diffraction pattern from the specimens with densely formed microcrystalline domains was explained on the basis of a large lattice distortion of microcrystals. A large amount of inclusion of boron atoms in α-iron precipitates in the early stage of precipitation at 350°C was found by the use of selected area electron diffraction.