Study of hydrogenation of Sm2Fe17-yGay by means of X-ray diffraction

The hydrogenation process of Sm2Fe17 - yGay(y=0-2) was studied. X-ray inves tigations shaw a decreasing hydrogen solubility in the intermetallic alloy with increasing Ga-content from 4.0+/-0.3 atoms per formula unit for Sm2Fe1 7 to 2.85+/-0.05 for Sm2Fe15Ga2. The larger Ga atoms reduce the size of the interstitial sites and thereby the maximum hydrogen concentration is decre ased. The behaviour of the lattice parameters a and c with increasing Ga co ntent points to a changed hydrogen distribution on the interstitial sites, becoming more statistical. In situ observations by means of high temperatur e X-ray diffraction show that the hydrogen absorption process is diffusion controlled. The hydrogen absorption starts at an annealing temperature of 1 20-140 degrees C in all cases. The solubility of hydrogen decreases with in creasing temperature. The hydrogen is completely desorbed above 350 degrees C in all cases. The absorption/desorption process is reversible between ro om temperature and 400 degrees C. Annealing at temperatures above 400 degre es C lends to the decomposition of the Sm2Fe17 phase, indicated by emerging of alpha-Fe. The formation of SmHx is established at 600 degrees C. The de composition temperature increases with increasing Ga-content. Up to 750 deg rees C, only Sm2Fe17 is completely decomposed. (C) 2000 Elsevier Science S. A. All rights reserved.