FACTORS CONTROLLING HYDROGEN-INDUCED AMORPHIZATION OF C15 LAVES COMPOUNDS

The radii of tetrahedral holes occupied by hydrogen atoms in the amorphous hydrides a-RFe2Hx (R=a rare earth metal) are larger than the empirical minimum hole size, 0.040 nm, of the general stable hydrides AxB1-xHy. However, the radii of the holes in the corresponding crystalline hydrides c-RFe2Hx are smaller than this' value. Thus, the higher stabilities of the amorphous hydrides which are considered to be the origin for the driving force of hydrogen-induced amorphization (HIA) in the C15 Laves compounds are interpreted on the basis of the differences in the hole sizes. Furthermore, the occurrence of HIA in C15 AB2 Laves compounds has been analyzed in terms of the tetrahedral hole size, the stability of the compounds, the atomic size ratio, the valence electron concentration and the contraction or the expansion of the constituent elements and the holes. The atomic size ratio is the single most important factor controlling the occurrence of HIA in these compounds, and the compounds with the ratio above 1.37 are amorphized by hydrogenation.