Hydrogen dynamics in the hydrides of Pr2Fe17 as revealed by Mossbauer spectroscopy

The rhombohedral Pr2Fe17Hx compounds with the Th2Zn17 structure have been p repared for x = 0-5. Their lattice parameters and Curie temperatures have b een determined from powder x-ray diffraction and thermomagnetic measurement s, respectively, and their Mossbauer spectra have been measured between 4.2 and 295 K. The Mossbauer 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 co mpounds. The Mossbauer spectra of Pr2Fe17H3 below 90 K, of Pr2Fe17H4 betwee n 4.2 and 295 K, and of Pr2Fe17H5 above 155 K, have been analyzed with a fo ur sextet model, indicative of an axial magnetization in these compounds ov er 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 coeff icients. A magnetic phase diagram for the Pr2Fe17Hx compounds is proposed o n the basis of their magnetic Mossbauer spectra. The Mossbauer spectra of P r2Fe17H5 indicate that, above 155 K, the two hydrogen atoms occupying one t hird of the tetrahedral 18g sites are rapidly jumping, on the Mossbauer tim e 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 hype rfine parameters of the Pr2Fe17Hx compounds indicates an initial filling of the interstitial 9e sites by the first three hydrogen atoms and then a sub sequent filling of the interstitial 18g sites by the last two hydrogen atom s. (C) 1999 American Institute of Physics. [S0021-8979(99)01216-5].