Nitrogenation of Sm 2Fe 17: mechanism, phases and stability

Abstract

The nitrogenation process was investigated in the reaction between Sm 2Fe 17 powders and nitrogen gas. The phases present during the process were determined by X-ray diffraction and thermomagnetic analysis. In the intermediate stages of the nitrogenation reaction a grain consists of three compositional regions: a shell of constant composition Sm 2Fe 17N 3 (saturated), a diffusion layer of continuous composition Sm 2Fe 17N x (0< x<3) and a core of a nitrogen-free Sm 2Fe 17. During the nitrogenation the diffusion layer moves inwards, with the Sm 2Fe 17N 3 shell growing and the Sm 2Fe 17 core shrinking. Eventually the diffusion layer reaches the centre of the grain and its composition becomes uniform throughout (Sm 2Fe 17N 3). Meanwhile a portion of the metastable Sm 2Fe 17N 3 compound undergoes decomposition (in a thermally activated manner) into α-Fe, SmN and N 2. It is suggested that the growth rate of Sm 2Fe 17N 3 is limited by the low mobility of the diffusion layer due to the low diffusivity of N atoms. The low diffusivity of the N atoms is attributed to the strong binding energy between the N and Sm atoms and the disadvantageous configuration of the 9( e) interstices in the crystal where the N atoms reside. The generally observed “lower than 3N/Sm 2Fe 17” overall composition is suggested to be due either to the incomplete nitrogenation of the sample or to the partial thermal decomposition of the Sm 2Fe 17N 3 into α-Fe, SmN and N 2. Reported values of “higher than 3N/Sm 2Fe 17” may be due to the stabilization of the Fe 4N phase during the decomposition of the Sm 2Fe 17N 3.