Emergence of order in nanocrystalline SmFe9

Three samples consistent with 1:12, 2:17 and 1:7 stoichiometry, ball-milled and subsequently annealed at temperature T-a between 600 and 1200 degrees C were studied by Rietveld analysis, coupled to Curie temperature T-c measu rements and Mossbauer spectroscopy. For all samples, at 600 degrees C less than or equal to T-a < 900 degrees C, the detected out-of-equilibrium phase is the hexagonal P6/mmm structure derived from TbCu7 with the composition SmFe9. At T-a greater than or equal to 900 degrees C the ordered R (3) over bar m phase of Th2Zn17-type is observed. With increasing T-a the unit cell parameter small variation of the parent SmFe9 induces a decrease of T-c an d [H-hf]. For T-a = 600 degrees C SmFe9 is defined by a = 0.4919 nm, c = 0. 4162 nm, T-c = 207 degrees C and [H-hf] = 232 kOe. The resulting ordered Sm -2 Fe-17 is characterized by a = 0.4941 root 3 nm, c = 3 x 0.4147 nm, T-c = 144 degrees C and [H-hf] = 219 kOe. For T-a = 750 degrees C the SmFe9 T-c is 171 degrees C associated to (H-hf) = 222 kOe. (C) 2000 Elsevier Science B.V. All rights reserved.