Phase diagram of hydrogen in palladium

Hydrogen in palladium, Pd-H(D), is an interesting system because of the highly mobile hydrogen and the presence of a phase boundary below 100 K. Experimentally, however, the nature of this transition has not been established. Historically this transition around 55 to 100 K has been thought to be an order-disorder transition. Such a transition would produce a phase boundary with anomalies at specific hydrogen concentrations corresponding to the specific ordered structures. In order to check this phase boundary we have performed a detailed study of the hydrogen concentration dependence of the specific heat of PdHx over the temperature range from 0.5 K to above 100 K using PdHx specimens with x up to 0.8753. The measured heat capacity has been analyzed as the sun of contributions due to the lattice specific heat of Pd, the electronic specific heat of PdHx, and the excess contribution caused by hydrogenation of the specimen. The excess specific heat result shows a sharp peak which indicates a phase boundary with transition temperature T-1 = 55 K to 85 K depending linearly on the hydrogen concentration from x = 0.6572 to 0.8753. We do not observe anomalies at specific x values as would be expected for the specific ordered structures.