Hydrogen absorption characteristics in Zr0.2Ho0.8CoFe

Hydrogen absorption studies have been carried out on the Zr0.2Ho0.8CoFe alloy which has the C15 cubic Laves phase structure. The data were collected in the temperature and pressure ranges 27≤T( C)≤150 and 0.05≤P(bar)≤45 respectively. A maximum hydrogen concentration of 9 hydrogen atoms per formula unit at 43 bar at RT is observed. The P-C isotherms show two plateau regions which are identified as the (α+β) and (β+γ) phases. These are confirmed from the variation of thermodynamic parameters as the function of hydrogen concentration and from the powder X-ray diffractograms of the samples taken at different hydrogen concentrations. The relative partial molar enthalpy and entropy of hydrogen are found to be in the ranges -(1-8) kJ (mol)-1 and -(13-22) JK-1(mol H)-1, respectively. The unit cell volume expansion (ΔV/V) up to 22% is found at the maximum hydrogen concentration at RT without a change in its crystal structure. The powder X-ray diffractogram of Zr0.2Ho0.8CoFeH9 does not indicate the occurrence of hydrogen induced amorphization (HIA) unlike the case of Zr0.2Ho0.8Co2H3 even though the atomic size ratio (RA/RB) of Zr0.2Ho0.8CoFe is more than the critical value estimated for the formation of an amorphous state upon hydrogenation. (orig.)