Abstract
The hydrogenation process of Sm 2Fe 17− y Ga y ( y=0–2) was studied. X-ray investigations show a decreasing hydrogen solubility in the intermetallic alloy with increasing Ga-content from 4.0±0.3 atoms per formula unit for Sm 2Fe 17 to 2.85±0.05 for Sm 2Fe 15Ga 2. The larger Ga atoms reduce the size of the interstitial sites and thereby the maximum hydrogen concentration is decreased. The behaviour of the lattice parameters a and c with increasing Ga content points to a changed hydrogen distribution on the interstitial sites, becoming more statistical. In situ observations by means of high temperature X-ray diffraction show that the hydrogen absorption process is diffusion controlled. The hydrogen absorption starts at an annealing temperature of 120–140°C in all cases. The solubility of hydrogen decreases with increasing temperature. The hydrogen is completely desorbed above 350°C in all cases. The absorption/desorption process is reversible between room temperature and 400°C. Annealing at temperatures above 400°C leads to the decomposition of the Sm 2Fe 17 phase, indicated by emerging of α-Fe. The formation of SmH x is established at 600°C. The decomposition temperature increases with increasing Ga-content. Up to 750°C, only Sm 2Fe 17 is completely decomposed.
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