College Park, Maryland June 6 - 10 , 2004
TP6: Neutron scattering studies of hydrogen dynamics in Pr2Fe17HX
E. Mamontov (University of Maryland, College Park; NIST Center for Neutron Research), T. J. Udovic (NIST Center for Neutron Research), O. Isnard (Institut Laue Langevin), J. J. Rush (NIST Center for Neutron Research)
Hydrogen has a profound influence upon the structural and magnetic properties of the fundamentally interesting R2Fe17 rare-earth compounds. Typical of such compounds, Pr2Fe17 crystallizes in the Th2Zn17 rhombohedral (R-3m) structure. Hydrogen can insert into Pr2Fe17 to form the hydrides Pr2Fe17HX. Neutron diffraction results indicate that the first three hydrogen atoms fully occupy the interstitial 9e octahedral (o) sites, whereas the last two hydrogen atoms occupy one-third of the available interstitial 18g tetrahedral (t) sites . Because the t-site hydrogen is believed to occupy only two diametrically opposed sites at the corners of a hexagon composed of six 18g sites , it is expected that the t-site hydrogen atoms in Pr2Fe17H4 and Pr2Fe17H5 would readily jump between the available 18g sites. Our quasielastic neutron scattering measurements confirm the presence of localized hydrogen motion in Pr2Fe17H3 and Pr2Fe17H5. The time between jumps is on the order of 100 ps at 200 K and decreases as temperature is increased. Neutron vibrational spectra of Pr2Fe17HX (x = 3, 5) and Pr2Fe17DX (x = 2, 3, 4, 5) are found to be consistent with the hydrogen locations determined by diffraction. For example, for Pr2Fe17H3, two peaks centered at ~105 meV and 85 meV correspond to the normal modes of the octahedral hydrogens in the ab plane and along the c axis, respectively. For Pr2Fe17H5, the c-axis mode for the o-site hydrogen softens considerably, in part due to the large lattice expansion along the c direction caused by the extra hydrogen occupying some of the t sites. In addition to the slightly softened ab-planar modes of the o-site hydrogen, there is extra scattering intensity at 110 meV and above due to the relatively higher-energy normal modes of the t-site hydrogen.
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