Our previous HRPD experiment on the Pd-D2 system resolved the long-standing controversy over the occupation of T interstitial sites by D, showing that T sites are occupied owing to passage over the thermodynamical critical point (39 bar D2 pressure at 283 degC). A concomitant effect is a significant excess D concentration when the sample is quenched to room temperature under D2 pressure, compared to formation of Pd-Dx by a phase transformation at room temperature. Very recently, we showed, using neutron diffraction, that the quenched deuteride is stable in vacuum or air. The latter phenomena are entirely novel in the long history of Pd-H2/D2 and this experiment aims to confirm and explore them authoritatively. We also aim to re-define the critical point directly, by observation of a single supercritical phase via crystallography, rather than by the disappearance of pressure hysteresis.