Crystal structure and magnetic properties of the UFe7Al5 uranium-iron aluminide

The ternary compound UFe7Al5 was synthesized by arc melting, followed by annealing at 850 deg. C. The crystal structure was determined by single-crystal X-ray diffraction and refined to a residual value of R=0.039 (S=1.030), with lattice parameters a=8.581(2) A and c=4.946(1) A. This compound is a new extreme composition in the family of intermetallics with general formula UFexAl12-x crystallizing in the tetragonal ThMn12-type structure, space group I4/mmm. In contrast to UFexAl12-x within the composition range 4≤x≤6, in UFe7Al5 the additional iron atom is found in the 8i equipositions. Magnetization measurements versus temperature show two magnetic transitions at 363 and 275 K, respectively, with a ferromagnetic behavior below the highest temperature transition. 57Fe Moessbauer data indicate that the high-temperature transition is related to the ordering of the iron atoms. The dependence of the isomer shifts and magnetic hyperfine fields on the crystallographic site and on the number of the iron nearest neighbors is similar to that observed in the other UFexAl12-x and rare-earth analogues. The magnetic hyperfine field values of iron atoms on 8i sites is larger than in the other sites, in agreement with previous data obtained for other ThMn12-type compounds