Condensed Matter Seminars

The behavior of hydrogen in metals has attracted much attention in fundamental and applied research areas. Palladium hydride (PdHx) is a typical metal-hydrogen system and has been studied for many decades. Pd has remarkable abilities to absorb plenty of H atoms and the H atoms are highly mobile in the Pd lattice. It is interesting to examine how the properties are changed as the particle size is reduced to a nanometer-scale. We have investigated structure, diffusion and vibrational dynamics by means of neutron scattering techniques for both bulk and nanocrystalline PdHx with a diameter of 8nm. Neutron diffraction work on nanocrystalline sample demonstrated that some of hydrogen atoms are accommodated at the tetrahedral (T) sites. This is in contrast to bulk PdHx with octahedral (O) occupation. In quasielastic scattering measurements, we found an additional fast diffusion process with a small potential barrier in nanocrystalline PdHx. Furthermore, our inelastic scattering works revealed that nanocrystalline PdHx exhibits two distinct vibrational excitations; one resembles that observed in bulk PdHx and the other is the excitations appeared at higher energies. The additional diffusion process and vibrational states are attributed to the H atoms at T sites near the surface of nanoparticles. The potential shape around the T site near the surface will be discussed in the seminar.