NEUTRON-DIFFRACTION AND MOSSBAUER SPECTRAL STUDY OF THE ND2FE16TICX SOLID-SOLUTIONS

Three samples of Nd2Fe16TiCx, 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 Mossbauer spe ctroscopy. Nd2Fe16Ti and Nd2Fe16TiC0.3 were synthesized by induction m elting stoichiometric amounts of the constituent elements, whereas Nd2 Fe16TiC2.8 was synthesized by methane-derived gas phase insertion of c arbon into finely ground Nd2Fe16Ti at 600 K. The neutron diffraction d etermined titanium site occupancies are similar in both Nd2Fe16Ti and Nd2Fe16TiC2.8 in which titanium preferentially occupies the 6c transit ion metal site. In contrast, the titanium occupancies in Nd2Fe16TiC0.3 are markedly different in that titanium avoids the 6c transition meta l site and randomly occupies the other three transition metal sites. T his difference in occupancies most likely occurs because the titanium diffusion rate during the quenching of Nd2Fe16TiC0.3 is affected by th e 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 temper ature of Nd2Fe16TiC2.8 is much lower than the 746 K Curie temperature of Nd2Fe17N3. This is an indication that the volume expansion, which o ccurs upon nitrogenation of R(2)Fe(17), is not the only factor which c ontributes to the increase in the Curie temperature. The Mossbauer spe ctra 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 i n Nd2Fe17. The 85 K Mossbauer spectrum of Nd2Fe16TiC0.3 is virtually i dentical to that of Nd2Fe17 and indicates an approximately random tita nium occupancy of the four transition metal sites. (C) 1996 American I nstitute of Physics.