EFFECT OF ALUMINUM ADDITIONS ON THE STRUCTURAL AND HYDROGEN ABSORPTION PROPERTIES OF AB5 ALLOYS WITH PARTICULAR REFERENCE TO THE LANI5-XALX TERNARY ALLOY SYSTEM

The dissociation pressures of the hydrides LaNi4.6Al0.4H6, LaNi4.5Al0.5H6, CFMNi4.8Al0.2H6 (CFM = cerium-free mischmetal), CFMNi4.6Al0.4H6 and YNi4AlH4 were studied. In all cases, the aluminum additions were found greatly to reduce the plateau pressure of the ANi5-x Alx hydride compared with that of the corresponding ANi5 hydride (A = La, CFM, Y). LaNi4.6Al0.4 and LaNi4.5Al0.5 were specially prepared homogeneous alloys whose enthalpies and entropies of transition were calculated as - 8.7 kcal (mol H2)-1, - 9.2 kcal (mol H2)-1 - 26.1 cal (mol H2)-1 deg-1 and 26.6 cal (mol H2)-1 deg-1, respectively. A crystallographic model of LaNi4Al is shown and consideration of the nearest neighbor atom distribution leads to a rationalization of the observed linear relationship between the enthalpy change ΔH and the Al composition. A recently proposed model of hydrogen interstitial site occupation is examined in the light of the present data. Finally, correlations of hydride dissociation pressures with either alloy heats of formation or alloy cell volumes are discussed. © 1979.