Dynamics of hydrogen in Pr 2 Fe 17 H 4 and Pr 2 Fe 17 H 5
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Dynamics of hydrogen in Pr2Fe17H4 and Pr2Fe17H5 Eugene Mamontov1,2, Terrence J. Udovic1, John J. Rush1, and Olivier Isnard3,4 1 NIST Center for Neutron Research, National Institute of Standards & Technology, 100 Bureau Dr., MS 8562, Gaithersburg, MD 20899-8562, U.S.A. 2 Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742-2115, U.S.A. 3 Laboratoire de Cristallographie, CNRS, associé à l’Université J. Fourier, BP 166X, F-38042 Grenoble Cedex, France 4 Institut Universitaire de France, Maison des Universités, 103 Boulevard Saint-Michel, F-75005 Paris, Cedex France ABSTRACT We describe a study of the dynamics of hydrogen in Pr2Fe17H4, where isolated hexagons formed by the interstitial tetrahedral (t) sites of the metal lattice are populated by mobile hydrogen atoms. An activation energy of 0.10 eV has been determined for a localized jump process involving the hopping of these hydrogen atoms among adjacent vertices of each hexagon, slightly lower than that for hydrogen hopping in Pr2Fe17H5, where each hexagon is occupied by two hydrogen atoms. INTRODUCTION The dramatic increase in Curie temperature and magnetization resulting from insertion of small atoms such as hydrogen into the host metal matrix of R2Fe17 makes these materials attractive candidates for hard magnet materials [1-3]. Among them, one of the most promising compounds is Pr2Fe17Hx [4-7]. Similar to other R2Fe17 compounds with lighter rare-earth atoms, Pr2Fe17 crystallizes in the Th2Zn17 rhombohedral R-3 m structure [8]. The host metal structure is retained in the hydride. Initially, hydrogen atoms in Pr2Fe17Hx fill interstitial 9e distorted octahedral (o) sites, with four Fe atoms and two Pr atoms at the corners [9], and at x = 3, all o sites become populated. Further hydrogenation leads to filling the interstitial 18g tetrahedral (t) sites, with two Fe atoms and two Pr atoms at the corners [8]. The t sites form arrays of isolated hexagons in the basal plane of the hexagonal structure normal to the c-axis, with a side dimension of ≈ 1.16 Å. The small size of the hexagons explains why the maximum hydrogen uptake in Pr2Fe17Hx corresponds to x = 5, with onethird of the available t sites in the Pr2Fe17 host structure filled. Isnard et al. [3] proposed that a pair of hydrogen atoms occupies two diametrically opposed vertices of the hexagons in order to comply with Switendick’s empirical criterion of ≈ 2.1 Å minimum H-H separation in ordered metal hydrides. Such a minimum has been ascribed to a repulsion interaction [10]. All but the opposite vertices of these hexagons in Pr2Fe17H5 are separated by less than 2.1 Å. We have suggested earlier [11] that the jumps of these two H atoms must be correlated in order to reconcile the dynamics of hydrogen hopping in Pr2Fe17H5 with the Switendick criterion. In the work described here, we have extended our earlier study to include the compound with a composition of Pr2Fe17H4. In this case, an isolated hexagon contains, on average, only one hydrogen atom, which allows
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