On the atomic diffusion mechanism and diffusivity of nitrogen atoms in Sm 2Fe 17

Abstract

An atomic diffusion mechanism (voidal diffusion) of nitrogen atoms in Sm 2Fe 17 is proposed which clearly explains the low values of diffusivity observed. The nitrogen atoms are located inside 9( e) octahedra which share Sm corners but no faces. Migration of nitrogen atoms 9( e) sites cannot occur by direct jumping. The most probable way for nitrogen atoms to migrate is by jumping from a 9( e) site into a thermodynamically unstable tetrahedral 18( g) site and subsequently into a new 9( e) site. In such a migration path a nitrogen atom has to surmount an enormous energy barrier representing the energy needed to overcome the strong bonding with its nearest neighbours (Fe and especially Sm atoms) and more importantly the strain energy 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 cannot accommodate any nitrogen atoms under equilibrium conditions, their presence plays a key role in the diffusion of nitrogen atoms. The present atomic diffusion mechanism predicts that the anisotropic ratio of the planar ( D xx ) to the axial ( D zz ) diffusivity should be equal to about 0.3. It also predicts that atoms such as hydrogen, which can occupy the 18( g) tetrahedra under equilibrium conditions, can jump inside the “circular tunnel” formed by the adjacent 18( g) tetrahedra, resulting in an increase in the planar diffusivity D xx , and consequently the anisotropic ratio D xx D zz can reach values as large as 1.07 (essentially isotropic behaviour).