Influence of the silicon composition on the crystallographic properties of the new ferromagnetic ternary silicides U2Fe17-ySiy (3.3 ≤ y ≤ 4.5) and U2Co15Si2

After melting and quenching, the ternary silicides U2Fe17-ySiy exist for 3.3 ≤ y ≤ 4.5 but they partially decompose after annealing at 850-900 degrees C. Their crystal structure, determined by X-ray diffractometry on both single crystals and powder, derives from the hexagonal Th2Ni17-type but depends strongly on the silicon content. For y = 3.7 some uranium sites are partially replaced by pairs of iron atoms and conversely. This structure shows some similarities to that observed for the binary compound Ho2Fe17. On the other hand, for y = 4.2, all the uranium atoms and pairs of iron atoms are statistically distributed. In contrast, the ternary silicide U2Co15Si2, which is obtained as single phase after annealing at 850 degrees C, adopts the Th2Ni17-type structure. In all compounds, iron or cobalt atoms of the pair are never substituted by silicon atoms. 13 refs., 6 figs., 4 tabs