Unusual Volume Reduction in CaLi_2-xMg_x by Hydrogen Uptake

抄録

To understand the hydrogen storage properties of CaLi_2-xMg_x having a C14 Laves phase structure, changes in its average and local structures by hydrogenation were investigated by using the Rietveld and the atomic pair distribution function analyses of X-ray diffraction data. We found that after hydrogen uptake at 283 K, Mg-containing samples exhibit a C14 phase with reduced volume. This C14 phase readily decomposes into CaH₂ at temperatures higher than 300 K. Our study suggests that hydrogen absorption in CaLi_2-xMg_x at 283 K is multi-step reaction. Lithium that can easily diffuse through C14 crystals reacts with hydrogen forming a Li-rich hydride phase. On the other hand, slow diffusion of large Ca atoms and strong Mg-Mg and Mg-Ca interaction restrain the immediate formation of CaH₂. As a result, there is a substantial time lag between the formation of a Li-rich hydride phase and the formation of CaH₂ at 283 K. This brings about a C14 phase with reduced volume where a large number of Li vacancies are present immediately after hydrogenation.