High-pressure synthesis and thermodynamic stability of PdH 1 ± ε up to 8 GPa

Palladium hydride alloys are superconductors and hydrogen storage materials. One synthesis route is compression of Pd to high pressure in a hydrogen-rich environment. Here we report the evolution of the unit cell volume of ${\mathrm{PdH}}_{x}$ synthesized by compressing Pd in a pure ${\mathrm{H}}_{2}$ medium to pressures from 0.2 to 8 GPa in a diamond anvil cell at room temperature. The volume of the face-centered cubic unit cell changes nonmonotonically with pressure, increasing upon compression from 0.2 to 1 GPa and decreasing upon compression from 1 to 8 GPa. Volume is reversible upon decompression and is independent of whether the sample was heated to 600 K at low pressure ($Pl2$ GPa). The x-ray diffraction data show no evidence for a phase transition between 0.2 and 8 GPa. The volume maximum at 1 GPa must be caused by progressive hydrogenation from 0 to 1 GPa. Assuming a pressure-volume-composition equation of state derived from previously published data, the [H]:[Pd] ratio in this study increases to a maximum value of $x=1\ifmmode\pm\else\textpm\fi{}0.02$ at $2\ifmmode\pm\else\textpm\fi{}0.5$ GPa and remains stable upon further compression to and from 8 GPa. These results add to a mounting body of evidence that ${\mathrm{PdH}}_{1\ifmmode\pm\else\textpm\fi{}\ensuremath{\epsilon}}$ is in thermodynamic equilibrium with pure ${\mathrm{H}}_{2}$ at room temperature from 2 GPa to at least 8 GPa. The simplest interpretation is that H atoms occupy all octahedral sites and no tetrahedral sites in face-centered cubic ${\mathrm{PdH}}_{1.0}$.