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TP6: Neutron scattering studies of hydrogen dynamics in Pr₂Fe₁₇H_X

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 Pr₂Fe₁₇H_X. 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 [1]. 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 [2], 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 Pr₂Fe₁₇H_X (x = 3, 5) and Pr₂Fe₁₇D_X (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.

[1] O. Isnard, S. Miraglia, J. L. Soubeyroux, D. Fruchart, and A. Stergiou, J. Less-Common Met. 162, 273 (1990).
[2] O. Isnard, S. Miraglia, J. L. Soubeyroux, and P. L'Heritier, J. Magn. Mater. 137, 151 (1994).

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