The thermodynamic parameters for the LaNi5-xAlx-H2 and MmNi5-xAlx-H2 systems

The thermodynamic parameters of the LaNi5-xAlx-H-2 and MmNi(5-x)Al(x)-H-2 (Mm=Mischmetal) systems have been investigated. The relative error of the thermodynamic parameter ratio Delta S/Delta H (entropy change/enthalpy change) is smaller than those of Delta H and Delta S determined from the van't Hoff plots. Delta S/Delta H shows a clear hyperbolic relationship with the crystallographic size. The singular point in the hyperbolic expression is related to the atomic radius of hydrogen through the preferred interstices for hydrogen atoms. The results obtained in these two systems are almost the same as those found in the ZrMn2-H-2 systems. (C) 1999 Elsevier Science S.A. All rights reserved.