A NEUTRON-DIFFRACTION AND MOSSBAUER SPECTRAL STUDY OF THE STRUCTURE AND MAGNETIC-PROPERTIES OF THE Y2FE14-XSIXB SOLID-SOLUTIONS

A neutron diffraction and Mossbauer spectral study of Y2Fe14-xSixB sho ws that silicon preferentially occupies the 4c, and to a lesser extent , the 8j1 sites in Y2Fe14-xSixB. The trend in the site occupancy is th e same as in Nd2Fe14-xSixB. The Curie temperature of Y2Fe14-xSixB incr eases with increasing silicon content. Neutron diffraction data show t hat the increase in Curie temperature is accompanied by a contraction of the unit cell. Wigner-Seitz cell calculations, using the Y2Fe14-xSi xB lattice and positional parameters obtained by neutron diffraction, show that the silicon site occupancy is correlated with the rare earth contact area of that site. The Mossbauer spectra of Y2Fe14-xSixB have been fit with a model which takes into account the distribution of ne ar-neighbor environments of an iron atom due to the presence of silico n. The weighted average of H(max), the average hyperfine field of the iron sites with no silicon near neighbors, decreases by 42 and 45 kOe per silicon substitution per formula unit at 85 and 295 K, respectivel y. The weighted average of DELTAH, the average reduction in the hyperf ine field caused by the addition of one silicon near neighbor to the n ear neighbor environment, is about 13.5 and 18 kOe at 85 and 295 K, re spectively, and does not show an appreciable dependence on the silicon content. The isomer shifts obtained from these fits suggest an increa se in covalency of bonding on silicon substitution, an increase which is consistent with the preference of silicon to bond with ttrium.