HYDROGEN-INDUCED AMORPHIZATION OF THE LAVES COMPOUND CENI2 AND THE STRUCTURAL AND THERMAL-CHARACTERISTICS OF THE AMORPHOUS PHASE

Abstract

The hydrogen-induced amorphization behaviour of the C-15 Laves compound CeNi2 were investigated by X-ray, differential scanning calorimetry, transmission electron microscopy and thermal desorption techniques. From the X-ray measurements, it was observed that the CeNi2 compound is amorphized by hydrogenation, and the transformation to an amorphous state takes place without the formation of a crystalline hydride phase. At high reaction temperatures (400-degrees-C), the sample decomposes into the CeNi5 and CeH2 phases, which are believed to be equilibrium phases in a hydrogen atmosphere. Amorphization is possible even when the reaction temperature is as low as -76-degrees-C. From this low temperature amorphization behavior, it is suggested that the elastic strain due to hydrogen absorption may play a key role in the amorphization. Electron diffraction of the hydrogenated sample shows two diffuse halos, which are proposed to be caused by phase separation of the amorphous phase. One of these is considered to be a cerium-rich phase (close to CeH2) and the other is a nickel-rich phase (close to CeNi5). Therefore, it is suggested that the amorphization of CeNi2 by hydrogenation may occur by lattice distortion in the course of the phase decomposition. The thermal decomposition and hydrogen desorption behaviors of the amorphous CeNi2 hydride was examined by differential scanning calorimetry and a thermal desorption technique using gas chromatography. The first crystallization product is the CeNi5 phase followed by the formation of CeH2 at higher temperatures. Through the formation of intermediate compounds of varying [Ce]/[Ni] ratios, the CeNi5 and CeH2 phases combine into the original CeNi2 compound.