Particle size dependence of the Pr₂Fe₁₇ nitrogenation process

Abstract

The particle size dependence of the Pr₂Fe₁₇ nitrogenation process at 400°C is determined by the Mössbauer method. The experimental data are described in terms of a particle model consisting of spherical concentrical shells containing: (i) an expanded central core saturated with 2+ɛ atoms N per formula unit; (ii) an intermediate unnitrided shell with a volume larger than the core by a factor of about 2.4 and accommodating a stress/strain field produced by the expanded core, and (iii) an unnitrogenated undeformed external shell. We observed that small particles (d ≤ 10 ώm and most probably lower) tend to absorb higher nitrogen contents than the large ones. Our results also indicate that some small particles, most probably single crystals, do not nitrogenate.