Thermal motion in palladium hydride studied by means of elastic and inelastic scattering of neutrons

A single crystal neutron diffraction investigation of electrolytically loaded palladium hydride of composition PdH _0.63 confirms the NaCl-type arrangement found previously for the material loaded from the gas phase. The root mean square displacements due to thermal motion of the palladium and hydrogen atoms at room temperature are found to be 0.10 ± 0.01 and 0.24 ± 0.02 Å, respectively. Measurements of the total neutron scattering cross section as a function of neutron energy and of the energy distribution of 0.004 eV neutrons scattered at 90° are reported and interpreted in terms of an Einstein model for the proton vibrations with a fundamental frequency corresponding to 0.056 ± 0.002 eV. From this the root mean square displacement of the protons relative to the palladium sublattice at room temperature is estimated to be 0.22 ± 0.02 Å, in agreement with the values found by diffraction. The cold neutron scattering data also give information about the heat motion of the palladium atoms. Interpreted on a Debye model they yield a Debye temperature of 300 ± 25°K and a root mean square displacement of 0.10 ± 0.01 Å.