Hydrogenation and nitrogenation of SmFe 2

Abstract

The characteristics of the reactions between SmFe 2 and H 2 or N 2 were studied by isochorothermal analysis, X-ray diffraction and thermomagnetic analysis. Under hydrogen pressures of below 150 kPa, SmFe 2 absorbs hydrogen instantaneously and exothermically at temperatures of between 120 and 160 °C. When hydrogenation occurs in an “excess of hydrogen gas”, the reaction products are dependent on the maximum hydrogenation temperature: unreacted SmFe 2, SmFe 2 hydride, amorphous (Sm,Fe 2) hydride, Sm hydride and α-Fe. When hydrogenation occurs in a “deficient hydrogen gas” atmosphere, Sm 2Fe 17, SmFe 3 and Sm are also found in the reaction products, depending on the maximum hydrogenation temperature. The amorphous (SmFe 2) hydride begins to decompose into an Sm-poorer amorphous (Sm,Fe 2) hydride and Sm hydride immediately after its formation. At about 325 cC, α-Fe also begins to crystallize and the composition of the amorphous phase varies continuously. The Curie temperatures of SmFe 2, SmFe 2 hydride and amorphous (Sm,Fe 2) hydride are 683 K, 110 K and 348–508 K (depending on the metal composition and hydrogen content) respectively. On heating under vacuum, the amorphous (Sm,Fe 2) hydride begins to lose hydrogen at temperatures of between 160 and 200 °C with simultaneous decomposition into Sm hydride and an Sm-poorer amorphous phase. The reaction of SmFe 2 powder with N 2 begins at about 250 °C with the formation of SmN, α-Fe and Sm. At high temperatures (950 °C), only SmN and α-Fe exist. When bulk SmFe 2 is used instead of powder, the reaction is not complete (even at 950 °C) and SmFe 3 begins to appear. No interstitial SmFe 2 nitride was formed at any temperature.