Entropies for hydrogen solution and hydride formation/decomposition in Pd alloys

Abstract For most fcc Pd alloys the relative standard partial entropy for hydrogen absorption, Δ S H ∘ , becomes more negative as the atom fraction of solute metal increases. Δ S H ∘ is the entropy change as the H concentration → 0 after removal of tne partial configurational entropy term assuming that all of the octahedral interstices can be occupied, as for pure Pd. This decrease in Δ S H ∘ is generally attributed to failure of this assumption due to the blocking of interstices adjacent to the solute metal atoms. On the other hand, the entropy changes for the dilute ⇄ hydride phase transitions for Pd alloys are generally unaffected by any selective occupation of interstices. The reason for this will be explored. By contrast with the effect of hysteresis on the enthalpy changes for the dilute⇄ phase transition reactions as measured from van't Hoff plots of the plateau pressures, the corresponding entropy changes are shown to be unaffected by hysteresis.