Abstract only.The phase diagram of PdH (and also PdD) has been studied extensively over the past century or more, and is considered to be well understood. However, there is a subtle issue in connection with the phase diagram this is not well understood; this has to do with the stability of the lattice itself, in connection with the different phases. In the literature, one usually finds the phase diagram for conditions under which the Pd sub-lattice is assumed to be fixed. Given the long relaxation time associated with vacancy diffusion under "normal" conditions, the phase diagram that results is very useful
