HYDROGENATION AND NITROGENATION OF SMFE3

The characteristics of the reactions between SmFe3 and H-2 or N2 were studied by isochorothermal analysis, X-ray diffraction and thermomagnetic analysis. SmFe3 absorbs about 3.1 atoms of hydrogen instantaneously at a temperature between 140 and 170-degrees-C. Subsequent cooling to room temperature causes an increase in the hydrogen concentration to about 4 hydrogen atoms per mole of SmFe3. As the temperature is increased, the SmFe3 hydride begins to desorb hydrogen and its composition becomes SmFe3H0.2 at about 350-degrees-C. The SmFe3 hydride desorbs and absorbs hydrogen reversibly in the temperature range 25-400-degrees-C, with a 16-degrees-C temperature hysteresis occurring between 249 and 233-degrees-C during heating and cooling. At about 425-degrees-C, the hydride begins to decompose into Sm hydride and alpha-Fe. The decomposition reaction is accompanied by a massive hydrogen absorption. The SmFe3 hydride has the rhombohedral crystal structure (same as the parent compound) with a 19% expanded lattice with parameters a = 5.378 angstrom and c = 27.345 angstrom. The stresses introduced by the huge lattice expansion cause the decrepitation of the SmFe3 hydride samples. The Curie temperature and saturation magnetization are 90-degrees-C and 70.8 e.m.u. g-1 for SmFe3H4 compared with 390-degrees-C and 80.6 e.m.u. g-1 for SmFe3. For both SmFe3 and SmFe3 hydride, the easy direction of magnetization is along the crystallographic c axis. SmFe3 powder reacts with N2 at about 250-degrees-C forming SmN, alpha-Sm and alpha-Fe. The final products are SmN and alpha-Fe. No interstitial SmFe3 nitride exists at any temperature.