Comparison of the dynamics of hydrogen and deuterium dissolved in crystalline Pd 9Si 2 and Pd 3P 0.8

Abstract

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.