Neutron-Diffraction and Mössbauer Effect Study of the Preferential Silicon Site Occupation and Magnetic Structure of Nd₂Fe₁₄₋ₓSiₓB

A neutron-diffraction study of Nd2Fe14-xSixB has shown that silicon preferentially occupies the 4c site in the transition-metal sublattice in Nd2Fe14B. Silicon also exhibits a moderate preference for the 8j1 site, is almost excluded from the 16k2 site, and avoids the 16k1, 8j2, and 4e sites. The silicon site occupancy is correlated with a preference for a silicon atom to have rare-earth atoms in its coordination environment. The Mössbauer spectra of Nd2Fe14-xSixB have been fit with a model which takes into account the distribution of near-neighbor environments of an iron atom due to the presence of silicon. These fits show that the substitution of silicon in the near-neighbor environment of an iron atom primarily influences the long-range contributions to the hyperfine field experienced by the iron. The mechanism for the increase in the Curie temperature when silicon is added to Nd2Fe14B-type magnets is more subtle than previously believed, but can be explained by the relative decrease in the proportion of short iron-iron bonds when silicon is substituted for iron