Hydrogen Dynamics in the Hydrides of Pr₂Fe₁₇ as Revealed by Mössbauer Spectroscopy


The rhombohedral Pr2Fe17Hx compounds with the Th2Zn17 structure have been prepared for x =0-5. Their lattice parameters and Curie temperatures have been determined from powder x-ray diffraction and thermomagnetic measurements, respectively, and their Mössbauer spectra have been measured between 4.2 and 295 K. The Mössbauer spectra for x=0, 1, and 2, obtained between 4.2 and 295 K, and those of Pr2Fe17H3, obtained above 90 K, have been analyzed with a seven sextet model, indicative of a basal magnetization in these compounds. The Mössbauer spectra of Pr2Fe17H3 below 90 K, of Pr2Fe17H4 between 4.2 and 295 K, and of Pr2Fe17H5 above 155 K, have been analyzed with a four sextet model, indicative of an axial magnetization in these compounds over the indicated temperature ranges. The axial magnetic anisotropy results from a combination of lattice expansion upon hydrogenation and contraction upon cooling, and the relative importance of the praseodymium Stevens coefficients. A magnetic phase diagram for the Pr2Fe17Hx compounds is proposed on the basis of their magnetic Mössbauer spectra. The Mössbauer spectra of Pr2Fe17H5 indicate that, above 155 K, the two hydrogen atoms occupying one third of the tetrahedral 18g sites are rapidly jumping, on the Mössbauer time scale of 100 ns, between the six available 18g positions, a jumping which slows down or ceases below 155 K. The compositional dependence of the hyperfine parameters of the Pr2Fe17Hx compounds indicates an initial filling of the interstitial 9e sites by the first three hydrogen atoms and then a subsequent filling of the interstitial 18g sites by the last two hydrogen atoms. © 1999 American Institute of Physics.



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© 1999 American Institute of Physics (AIP), All rights reserved.