Hydrogenation-induced microstructure changes of pseudo-binary (Pr sub(x)Mg sub(1-x))Ni sub(2) Laves compounds

Using transmission electron microscopy (TEM), we systematically investigate hydrogenation-induced microstructural changes of pseudo-binary (Pr sub(x)Mg sub(1-x))Ni sub(2) Laves compounds varying x from 0.3 to 1.0, which lead to the averaged constituent atomic-size ratio R sub(A)/R sub(B) ranging 1.34-1.47 of the supposed AB sub(2) compound. It is empirically known for the AB sub(2) Laves compounds that hydrogen-induced-amorphization (HIA) takes place when the R/R sub(B) exceeds 1.37. We find that, based on careful analyses of electron diffraction patterns and TEM images, the hydrogenation-processed microstructures of the (Pr sub(x)Mg sub(1-x))Ni sub(2) compounds exceeding the critical ratio are not pure amorphous but composed of Ni nano-crystals embedded in an amorphous matrix of hydride PrH sub(2). This provides a direct evidence of hydrogenation-induced micro-phase separation (HIMPS), and accordingly it is suggested that HIA believed so far should be attributed to HIMPS phenomenon.