Neutron scattering studies of the structure and dynamics of the PdCu-H ordered phase produced under a high hydrogen pressure

An atomically ordered PdCuH_{approximately=0.9} hydride was synthesized at 600 K and a hydrogen pressure of 3 GPa. Its metal lattice had a primitive tetragonal cell formed from the initial FCC cell of the disordered PdCu alloy due to ordering of the Pd and Cu atoms into alternate layers perpendicular to the tetragonal c-axis. Rietveld refinement of the neutron diffraction pattern of PdCuH_{approximately=0.9} measured at 80 K showed the H atoms to occupy octahedral interstitials within the Pd layers. The inelastic neutron scattering study at 35 K revealed a broad hydrogen optical peak with peculiarities at energy transfers of 79 and 95 meV and of 113 meV which were ascribed to the H vibrations in the Pd-H planes and along the Cu-H chains perpendicular to the planes, respectively. The potential for hydrogen atoms in the PdCuH_{approximately=0.9} ordered phase was thus noticeably different, even in the Pd-H plane, from that in the known hydrides of palladium and palladium alloys. The value of approximately 116 meV was predicted for the local H vibrations in dilute Cu-H solid solutions.