Thermodynamic and structural aspects of hydrogen absorption in the Zr(Al_xCo_1-x)₂ system

Hydrogen absorption and desorption isotherms have been measured for Zr(Al_xCo_1-x)₂ (0.1≤x≤0.5) compounds at pressures up to 60 atm and temperatures between -30 and 100 °C. Enthalpies and entropies of hydride formation have been derived from desorption isotherms for x=0.1, 0.167, 0.25 and 0.35. Below these x values the pressures, necessary for hydrogen absorption, are higher than 60 atm. The critical temperatures for hydride formation of the x=0.35 compounds are lower than the experimental temperatures employed in the present work. In general, the hydride stability increases while the critical temperature decreases as a function of x. Consequently, we suggest that a competition between two opposing factors, namely the metal-hydrogen interaction and the hydrogen-hydrogen interaction (or hydrogen-induced long range elastic strain interaction), accounts for the anomalous hydrogen sorption behavior in this and other Al-containing systems. The hydrogen equilibrium pressure for ZrCo₂ is estimated by extrapolation. The estimation compares fairly with available experimental information. The volume occupied by a single hydrogen atom is obtained for all the investigated compounds, and supports the idea of approximately constant value for this quantity.