NITROGEN DIFFUSION IN R(2)FE(17) LATTICE - A TRAPPING DIFFUSION PROCESS

X-ray diffraction, thermoconductivity and nuclear magnetic resonance e xperiments have been carried out in order to study the diffusion mecha nism of N atoms in the Y2Fe17 lattice. These experiments reveal that a small number of N atoms which enter the Y2Fe17 lattice, but do not oc cupy the octahedral interstitial sites, are mobile at the nitrogenatio n temperature, while most of the N atoms, which enter the octahedral s ites, are immobilized. Based on the characteristics of nitrogenation o bserved in R(2)Fe(17) (were R is the rare-earth element) systems, a tr ipping diffusion model has been proposed and discussed in detail. The previously proposed free diffusion model is the high-temperature appro ximation of the trapping diffusion model. The experiments and theoreti cal analysis show that it is the N-lattice interaction, instead of N-N interaction, that leads to the formation of the observed nitrided-unn itrided configuration, causes a tremendous decrease of the apparent di ffusion frequency factor and is responsible for the N uptake stability and irreversibility. Also, this work shows how the N-lattice interact ion affects the N uptake in various nitrogenation conditions.