Thermodynamic and structural aspects of hydrogen absorption in the Zr(Al_xFe_1-x)₂ system

Hydrogen desorption isotherms have been measured for Zr(Al_xFe_1-x)₂ (0.12≤x≤0.4) compounds at pressures up to 60 atm and temperatures between -30°C and 140°C. The enthalpies of hydride formation, derived from the temperature variation of the equilibrium pressures for x=0.12, 0.167, 0.2 and 0.4, show an increase of the hydride stability with increase of x. The shapes of the isotherms in the plateau regions indicate decrease of the critical temperature for the metal-to-hydride transition as a function of x. These two features closely resemble the observations for the recently investigated Zr(Al_xCo_1-x)₂ system and support further a proposed explanation for the anomalous hydrogen sorption behavior in these and other Al-containing intermetallics. No hysteresis between absorption and desorption isotherms was observed for the x=0.12 and 2/12 compounds. The lattice constants of the expanded crystal structures upon hydrogenation have been determined by employing X-ray diffraction.