Thermodynamic and hydrogen-induced structural properties of Ho <inf>1-x</inf>Mm<inf>x</inf>Co<inf>2</inf>-hydrides

Thermodynamic and hydrogen-induced structural properties of hydrogenated Ho1-xMmxCo2 (x ≤ 0, 0.1, 0.2, 0.3 and 0.4, Mm ≤ mischmetal) alloys have been investigated. The hydrogen absorption pressure-composition isotherms were studied in the temperature range 50-200 °C and pressure range 0.001-1 bar using a Sieverts-type apparatus. The effect of Mm on the equilibrium plateau pressure and hydride stability has been discussed. Structural analyses and hydrogen-induced amorphization of Ho 1-xMmxCo2hydrides were performed using powder x-ray diffraction. The structural stability of these alloys upon repeated hydrogenation and dehydrogenation cycles has been investigated. At lower hydriding pressure and temperatures, Ho1-xMmxCo 2-hydrides retain their Laves C15 structure; however, the large lattice volume expansion about 24% is observed in the β-phase. The effect of hydrogenation pressure and temperature and Mm concentration on the hydrogen-induced transformation from crystalline Ho1-xMm xCo2-H to amorphous Ho1-xMmxCo 2-H and recrystallization into elemental hydrides have been studied and discussed. © 2007 IOP Publishing Ltd.