Comparison of the dynamics of hydrogen and deuterium dissolved in crystalline Pd ₉Si ₂ and Pd ₃P _0.8

The bonding potentials of hydrogen and deuterium dissolved in the crystalline alloys Pd ₉Si ₂ and Pd ₃P _0.8 have been studied by neutron scattering techniques. Neutron powder diffraction verifies that, in both alloys, the principal type of interstitial absorption site is the quadrilateral base of a Pd-defined pyramid situated on the face of an empty triangular prism. The neutron vibrational spectroscopy results indicate that, although the H and D vibrational energies parallel to the Pd pyramidal base appear to be similar for both alloys, the H and D vibrational energies perpendicular to the Pd pyramidal base for Pd ₉Si ₂ are ≈15% softer than the corresponding energies for Pd ₃P _0.8. Both neutron vibrational spectroscopy and quasielastic neutron scattering show that hydrogen hopping between the two pyramidal absorption sites associated with different faces of the same prism is much more rapid in Pd ₉Si ₂ than in Pd ₃P _0.8. This may be directly attributable to the observed softer potential along the hopping trajectory (i.e. perpendicular to the Pd pyramidal base) for the interstitial site in Pd ₉Si ₂ compared to that in Pd ₃P _0.8.