CRYSTAL-STRUCTURE OF THE URANIUM MONOSILICIDE USI

The crystal structure of the uranium monosilicide USi (a = 10.587(3), c = 24.310(5) Angstrom, space group I4/mmm) has been determined from s ingle crystal X-ray diffraction data. It is of a new type, with 68 ura nium atoms per unit cell, distributed in 4e, 8f, 8j, 16n, 16n, and 16m positions. USi does not order magnetically at low temperature, and th e previously reported ferromagnetic ordering at T-c = 125 K is due to an oxygen-stabilised compound USiOx (x similar to 1/8) formerly known as an FeB-type uranium monosilicide.