STRUCTURAL-CHANGE DURING AMORPHIZATION AN D PHASE-SEPARATION PROCESSES IN THE YNI2-H SYSTEM

In order to investigate the processes of amorphization and phase separ ation in the YNi2-H system, we studied the feature of structural chang es in the YNi2-H system by X-ray diffraction method and thermal analys is as a function of hydriding temperature and time under a hydrogen pr essure of 1 MPa. From the X-ray diffraction and differential thermal a nalysis, it was demonstrated that the amorphization reaction for hydri ding at 473 K is much delayed in comparison with those at 573 and 273 K. This temperature dependence of the formation rate of the amorphous phase can be qualitatively explained by considering two promoting fact ors for Ni-diffusion, the lattice expansion by hydrogen absorption and thermal activation. We could also clarify that yttrium-rich tetrahedr al sites (3Y + 1Ni) increase with increasing hydriding temperature and time by analyzing the differential thermogravimetric data. In particu lar, when the hydriding temperature is high enough, the yttrium-rich s ites in the amorphous phase drastically increase. This suggests that t he growth of yttrium-rich and nickel-rich clusters occurs in the amorp hous structure, which may be recognized as a pre-phenomenon of the pha se separation into YHx and YNi3+delta phases.