Fe57Mössbauer studies ofNd2Fe14B andNd2Fe12.5Si1.5B (T=77–300 K)

${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer effects of ${\mathrm{Nd}}_2$ ${\mathrm{Fe}}_{14}$B and ${\mathrm{Nd}}_2$ ${\mathrm{Fe}}_{12.5}$ ${\mathrm{Si}}_{1.5}$B have been studied in a temperature range of 77–300 K. The distribution of silicon atoms over the six sites and the dependences of ${\mathit{H}}_{\mathrm{hf}}$, Δ, and δ on the temperature for the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ isotope have been deduced by fitting the Mössbauer spectra. The results show that about 80% of the silicon atoms occupy the ${\mathit{k}}_2$ site in ${\mathrm{Nd}}_2$ ${\mathrm{Fe}}_{12.5}$ ${\mathrm{Si}}_{1.5}$B. Meanwhile, silicon substitution for iron raises the Curie temperature and reduces the temperature coefficients of the hyperfine fields at the six iron sites, especially the ${\mathit{j}}_1$ site. The results are interpreted in terms of a decrease of the magnitude of the negative exchange interaction between the ${\mathit{j}}_1$-${\mathit{k}}_2$ atoms.