HYDROGEN ABSORPTION CHARACTERISTICS IN ZR0.2HO0.8COFE

Hydrogen absorption studies have been carried out on the Zr0.2Ho0.8CoF e alloy which has the C15 cubic Laves phase structure. The data were c ollected in the temperature and pressure ranges 27 less than or equal to T(degrees C) less than or equal to 150 and 0.05 less than or equal to P(bar) less than or equal to 45 respectively. A maximum hydrogen co ncentration of 9 hydrogen atoms per formula unit at 43 bar at RT is ob served. The P-C isotherms show two plateau regions which are identifie d as the (alpha+beta) and (beta+gamma) phases. These are confirmed fro m the variation of thermodynamic parameters as the function of hydroge n concentration and from the powder X-ray diffractograms of the sample s taken at different hydrogen concentrations. The relative partial mol ar enthalpy (<Delta(H)over bar (H)>) and entropy (<Delta(S)over bar (H )>) of hydrogen are found to be in the ranges -(1-8) kJ (mol H)(-1) an d -(13-22) J K-1 (mol H)(-1), respectively. The unit cell volume expan sion (Delta V/V) up to 22% is found at the maximum hydrogen concentrat ion 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 Zr0.2Ho0.8Co2H 3 even though the atomic size ratio (R-A/R-B) of Zr0.2Ho0.8CoFe is mor e than the critical value estimated for the formation of an amorphous state upon hydrogenation. (C) 1998 Elsevier Science S.A.