GENERAL FEATURES OF HYDROGEN-INDUCED AMORPHIZATION IN RM2 (R = RARE-EARTH, M = TRANSITION ELEMENT) LAVES PHASES

From a literature survey, the hydrogen-induced amorphization behaviour in RM2 (R = rare earth, M = Ni, Fe, Co) Laves phases is classified into two groups according to whether a crystalline hydride phase appears during the transition to an amorphous state or not. The major factor that determines the presence or non-presence of the crystalline metal hydrides is related to the stability of the compound, which can be estimated by the heat of formation (DELTA-H(f)) or the decomposition temperature of the compound itself. The crystalline metal hydrides are found only in the compounds with high stability. It is proposed that the mechanism for the two types of amorphization is related to the degree of lattice expansion with hydrogen absorption, which is a function of the elastic modulus of the compounds. To explain the dependence of the elastic modulus of the compound on the two types of amorphization behaviour, the concept of a nucleation barrier for amorphization is adopted. Compounds with a large lattice expansion (compounds with a low Young's modulus) do not produce the crystalline metal hydrides, which is attributed to amorphization without a nucleation barrier because the packing density of the crystalline state is lower than that of the corresponding amorphous state. Only the compounds with a small lattice expansion exhibit the crystalline state owing to a somewhat high nucleation barrier.