Formation of modified TbCu7 and Th2Zn17 type structures during annealing of mechanical-alloyed Sm-Fe powders

Compounds with the nominal composition near Sm2Fe17 were prepared by mechanical alloying starting from the elemental powders and subsequent annealing at temperatures, T-A, between 600 degrees C and 900 degrees C. For crystal structure investigations of the non-equilibrium phases formed at various temperatures, XRD methods with following Rietveld analysis were applied. For T-A between 600 degrees C and 750 degrees C a modified TbCu7-type structure of space group P6/mmm was found, in which the Fe(2c) site is replaced by the partially (1/3) and randomly occupied Fe(6l) site. Its approximate composition is SmFe8.8-9.0. For T-A between 800 degrees C to 900 degrees C a disordered modified Th2Zn17 structure (space group R (3) over bar m) was found that is formed by introducing additional randomly occupied Fe (6c) and Sm(3a) positions, respectively. The degree of order of Sm atoms and Fe-dumbbells along the c-direction increases with increasing T-A. A decrease in the Fe concentration in the cell is observed for increasing T-A. The completely ordered stoichiometric Th2Zn17-type structure could not be reached by annealing samples prepared from the ball-milled elemental powders. (C) 1998 Elsevier Science S.A.