In order to investigate the mechanism and characteristics of hydrogen-induced amorphization of $RM_2$ (R=rare earths, M=transition elements) Laves phases, the structural changes of samples hydrogenated at various conditions are examined by X-ray diffraction test and TEM (Transmission Electron Microscopy). The thermal behaviors of amorphous specimens are looked into through hydrogen thermal analysis using GC (Gas Chromatography) and DSC (Differential Scanning Calorimetry).
$CeNi_2$ and $CeFe_2$ which have C15 cubic structure can be amorphized at subzero temperatures, which indicate the importance of the elastic strain during hydrogenation on the disorder of the crystalline lattice. The above compounds cannot accommodate the elastic strain caused by hydrogen absorption. Instead, they can lower the strain energy through converting to amorphous state which has a loosed packing density than crystalline state. From the results of RDF (Radial Distribution Function) analysis on amorphous phases, it is found that hydrogen atoms are located on the tetrahedral sites surrounded by 4 Ce atoms so that the free energy of the system can be lowered. The microstructure of a-$CeNi_2H_{4.4}$ is observed to be composed of two phases, one is Ce-rich and the other is Ni-rich phase. But a-$CeFe_2H_x$ phase has a rather uniform compositional variance in the matrix. The crystallization processes of two alloys slightly differ from each other. The amorphous $CeNi_2H_{4.4}$ is decomposed into $CeH_2$ and $CeNi_5$, which the latter product has a highest stability in Ce-Ni binary system. And the amorphous $CeFe_2H_x$ phase is crystallized to binary Ce hydride and elemental Fe. However the crystallization process of two system is identically controlled by the movement of the transition elements.
Laves compounds $ErMn_2$ and $NdMn_2$ with C14 structure do not reveal any amorphization phenomena in this experimental conditions. But ternary compounds substituted with third elements such as $Er(Mn_{0.85}Co_{0.15})_2$ and $(Gd_{0.5}Nd_{0.5})Mn_2$ can be possible to amorphization even though they have C14 structures. Thus it can be said that hydrogen-induced amorphization of $RM_2$ Laves phases is not related to the crystal structure.
$ErCo_2$ alloy, which has C15 structure, is transformed to amorphous state during hydrogenation at 300$^\circ$C and 50 $\mbox{atmH}_2$ for 48hr after forming a crystalline hydride phase. At higher reaction temperature, it decomposes into $ErH_2$ and elemental Co phases. The structural changes of $ErCo_2$ during hydrogen absorption resemble with other Er-based compounds such as $ErFe_2$. In order to investigate the amorphization behaviors of Laves phases, hydrogen absorption tests are performed on the pseudo-binary compounds $Er(Co_{1-x}Mn_x)_2$, x=0-0.8, and $Er(Co_{1-x}Fe_x)_2$, x=0-1.0. The amorphization temperature, which is defined as the temperature amenable to the full amorphization in a fixed charging time (24hr) and pressure (50 $\mbox{atmH}_2$), gradually decreases as the fraction of Mn or Fe increases. The amorphization temperature is found to be proportional to the activation energy. The activation energy for the reaction is suggested to be dependent on the heat of formation ($\triangle$Hf) and the bulk modulus (B) of the compound. It increases as the bulk modulus is higher and the heat of formation is lower.
A Laves compound $CeRu_2$ with C15 structure, which has a forth period transition element, does not exhibit amorphization reaction during hydrogen absorption in this experimental conditions. But $CeRu_2H_5$ hydrogenated at room temperature is transformed to an amorphous state by partial hydrogen desorption at below 200$^\circ$C. This transformation is possible at room temperature. After the hydrogen desorption thoroughly, the sample converts to its original compound. To examine this abnormal and unreported phenomenon, hydrogenation treatments are performed by varying the alloy composition. The hydrogen absorption or desorption behaviors of $(Ce_{1-x}Gd_x)Ru_2$, x=0-0.4, are linearly changed with the Gd contents. At small Gd contents up to x=0.2, amorphization reactions by the partial hydrogen desorption are observed. But as the fraction of Gd increases, crystalline phases corresponding to original compounds are found. From the observed results, it is proposed that the structural modification of $CeRu_2H_5$ during hydrogen desorption is caused by the combined effects of valence change of Ce atoms and hydrogen desorption. The dramatic volume change of the compound due to the hydrogen desorption and volume contraction of the Ce atom may induce a disordered metastable amorphous state. The disordered state can be maintained because the reaction temperature is not high so that the movement of metal atoms is kinetically prohibited.
Substitution of Ni for Ru in $CeRu_2$ by small contents results in the change of the hydrogen absorption behavior so that amorphization occurs during hydrogen absorption at room temperature. By small substitution, the lattice distortion of the compound greatly increases. Therefore it is suggested that the amorphization of $Ce(Ru_{0.75}Ni_{0.25})_2$ during hydrogen absorption at room temperature is triggered by elastic instability such as $CeNi_2$. The amorphous $Ce(Ru_{0.75}Ni_{0.25})_2H_x$ phase is crystallized by hydrogen desorption at some elevated temperature to its original state through one step reaction of hydrogen desorption. Using X-ray technique, the critical elastic strain for the amorphization can be measured and it is about 1.9 vol\%, which value is very similar to the reported one in another intermetallic system.
