Hydrogenation of Sm 2Fe 17

Abstract

The characteristics of the reaction between Sm 2Fe 17 and H 2 have been studied by means of isochorothermal analysis, X-ray diffraction and thermomagnetic analysis. Sm 2Fe 17 begins to absorb hydrogen at about 185 °C. At 250 °C the hydrogen content reaches the maximum value of 2.4 atoms of hydrogen per mole of Sm 2Fe 17. Subsequent cooling to room temperature causes an increase in the hydrogen concentration up to the value of 5 H atoms per mole of Sm 2Fe 17. Above 250 °C the Sm 2Fe 17 hydride begins to desorb hydrogen and its composition becomes Sm 2Fe 17H 0.1 at about 500 °C. The Sm 2Fe 17 hydride desorbs and absorbs hydrogen reversibly in the temperature range between 25 and 500 °C. At about 510 °C the hydride begins to decompose into Sm hydride and α-Fe. The decomposition reaction is accompanied by massive hydrogen absorption. The Sm 2Fe 17 hydride has the rhombohedral crystal structure (the same as the parent compound) with a 3.4% expanded lattice with parameters α = 8.667 A ̊ and c=12.520 A ̊ . The stresses introduced by the lattice expansion cause crack formation along the Sm 2Fe 17 grain boundaries and the samples become friable. The Curie temperature and saturation magnetization are 280 °C and 159 e.m.u. g −1 for Sm 2Fe 17H 5 as compared with 135 °C and 116 e.m.u. g −1 for Sm 2Fe 17. For both Sm 2Fe 17 and Sm 2Fe 17 hydride the easy direction of magnetization is perpendicular to the c direction in the rhombohedral structure.