A theoretical interpretation of the pressure–composition isotherms of RNi5 (R=LaR=La, Pr, Nd and Sm) systems based on statistical mechanics

A simple model to understand the phase behavior of RNi5–H (R=LaR=La, Pr, Nd and Sm) systems is proposed based on the statistical mechanics. The essential parameters in the present model, hydrogen site energies and lattice relaxation term, were optimized so as to reproduce the experimentally determined pressure–composition isotherms (PCIs) of these four alloys. The qualitative difference in the PCI of LaNi5 from other alloys – at room temperature the former tends to show a single plateau while the latter have two plateaux – is explicable in terms of hydrogen site energies as follows. In case of LaNi5, four hydrogen sites, which are considered to belong to 3f, 12n, 6m and 12o   sites, are stable in αα phase, leading to a wide single plateau upto N∼6N∼6 where N is in RNi5HN below room temperature. At higher temperature, small differences in site energies for these sites brings the appearance of the second plateau. In case of other three alloys, on the other hand, the 6m site (or 12o) is less stable than LaNi5 in the αα phase. After obtaining enough lattice relaxation energy by around N=4N=4 through hydrogenation, the 6m   site becomes favorable for hydrogen, bringing the sencond plateau spanning from N∼4N∼4 to N∼6N∼6. Predicted T–C phase diagrams are also shown.