57Fe Moessbauer studies of Nd2Fe14B and Nd2Fe12.5Si1.5B (T=77--300 K)

57Fe Moessbauer effects of Nd2Fe14B and Nd2Fe12.5Si1.5B have been studied in a temperature range of 77--300 K. The distribution of silicon atoms over the six sites and the dependences of Hhf, Δ, and δ on the temperature for the 57Fe isotope have been deduced by fitting the Moessbauer spectra. The results show that about 80% of the silicon atoms occupy the k2 site in Nd2Fe12.5Si1.5B. 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 j1 site. The results are interpreted in terms of a decrease of the magnitude of the negative exchange interaction between the j1-k2 atoms