From partition function to phase diagram - statistical thermodynamics of the LaNi5-H system

The 6m sites of the LaNi5 lattice are not fully occupied by hydrogen at moderate pressures due to a blocking effect of the very closely spaced sites. It is usually not possible to treat this effect rigorously in continuous lattices. In this case, however, the 6m sites form isolated hexagons, for which a partition function can be constructed. The calculated absorption isotherms and the phase diagram display the observed features of a wide plateau region, which splits into two narrow plateaus above 338 K, and an additional plateau at pressures above 1000 bar. (orig.)