Three samples of , with x equal to 0.0, 0.3, and 2.8, with the Th2Zn17-type rhombohedral structure, have been studied by powder x-ray and neutron diffraction, magnetic measurements, and Mössbauer spectroscopy. Nd2Fe16Ti and Nd2Fe16TiC0.3 were synthesized by induction melting stoichiometric amounts of the constituent elements, whereas Nd2Fe16TiC2.8 was synthesized by methane-derived gas phase insertion of carbon into finely ground Nd2Fe16Ti at 600 K. The neutron diffraction determined titanium site occupancies are similar in both Nd2Fe16Ti and Nd2Fe16TiC2.8 in which titanium preferentially occupies the 6c transition metal site. In contrast, the titanium occupancies in Nd2Fe16TiC0.3 are markedly different in that titanium avoids the 6c transition metal site and randomly occupies the other three transition metal sites. This difference in occupancies most likely occurs because the titanium diffusion rate during the quenching of Nd2Fe16TiC0.3 is affected by the presence of carbon in the melt. Even though the unit cell volume of Nd2Fe16TiC2.8 is larger than that of Nd2Fe17N3, the 615 K Curie temperature of Nd2Fe16TiC2.8 is much lower than the 746 K Curie temperature of Nd2Fe17N3. This is an indication that the volume expansion, which occurs upon nitrogenation of R2Fe17, is not the only factor which contributes to the increase in the Curie temperature. The Mössbauer spectra of Nd2Fe16Ti confirm the high preferential titanium occupancy of the 6c site. At 85 K the weighted average hyperfine field of Nd2Fe16Ti is approximately 263 kOe, a value which is 33 kOe smaller than that in Nd2Fe17. The 85 K Mössbauer spectrum of Nd2Fe16TiC0.3 is virtually identical to that of Nd2Fe17 and indicates an approximately random titanium occupancy of the four transition metal sites.



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