Comparative Mössbauer Effect Study of Several R₂Fe₁₇ and R₂Fe₁₇Nₓ Compounds

The Mössbauer spectra of Sm ₂ Fe ₁₇ and Ho ₂ Fe ₁₇ and their nitrides have been measured between 295 and 85 K and analyzed with a model which is consistent with our earlier work on R ₂ Fe ₁₇ and R ₂ Fe ₁₇ N _x compounds, where R is Pr, Nd, and Th. This model is completely consistent throughout these rare-earth compounds and is in agreement with the crystallographic changes occurring upon nitrogenation and with the prediction of band structure calculations. The dramatic increase in Curie temperature in the nitrides results from the expansion of the crystallographic lattice, an expansion which is mainly centered on the 9d and 18h iron sites as is indicated by the increase of their Wigner-Seitz cell volumes upon nitrogenation. The 9d and 18h sites show a larger enhancement of their hyperfine fields as compared to the 6c and 18f sites as a result of improved ferromagnetic exchange between these sites and their near neighbors because of the lattice expansion and the consequent reduced iron 3d-iron 3d overlap.