It was noted that the preparation of Fe17Sm2N3 involved the slow diffusion of N atoms from the surface and into the bulk of the material. The atomic diffusion mechanism which operated in this case was voidal diffusion. The N atoms were located inside 9(e) octahedra, which shared Sm corners but had no faces between them. The N atoms migrated by jumping from a 9(e) site and into a thermodynamically unstable tetrahedral 18(g) site, and then into a new 9(e) site. Along such a migration path, the N atoms had to contend with an enormous energy barrier. This accounted for the energy which was required in order to overcome the strong bonding to its nearest neighbors (Fe and Sm atoms) and for the strain energy that was needed to break out through the octahedral face, Fe(f)-Sm(c)-Fe(h), and in through the tetrahedral face, Fe(h)-Sm(c)-Fe(h). Although the 18(g) sites could not accommodate N atoms in an equilibrium fashion, their presence played a key role in the diffusion of the N atoms. The diffusion of N atoms was anisotropic, as a result of the anisotropic crystal structure of Fe17Sm2. Bitter domain patterns were used to map the N diffusion fields of nitrogenated particles, and clearly revealed the expected anisotropic behavior of the diffusivity. The open structure of grain boundaries provided free paths, and behaved in essentially the same way as free surfaces did when exposed to N gas. The presence of H facilitated N diffusion, but fractured the particles so that N effectively penetrated towards the center of the particles. The use of NH3 gas caused severe morphological changes to the grains and resulted in the development of fine microstructures.

C.N.Christodoulou, N.Komada: Journal of Alloys and Compounds, 1995, 222, 27-32