CRYSTAL AND MAGNETIC-STRUCTURE OF THE URANIUM DIGERMANIDE UGE2

The crystal structure of the uranium digermanide UGe2 was determined a nd refined from single crystal X-ray diffraction data to R = 0.040, Rw = 0.052. UGe2 crystallizes with the orthorhombic ZrGa2 type (Cmmm) in stead of the ZrSi2 type (Cmcm) as was previously assumed on the basis of rather old and unreliable results. Our magnetic study of a polycrys talline are-melted UGe2 sample confirms the onset of ferromagnetic int eractions below ca. 52 K. Neutron powder diffraction studies show that the structure is collinear ferromagnetic, with uranium magnetic momen ts parallel to the a-axis of the orthorhombic cell; at T = 1.4 K, the uranium magnetic moment value is M(U) = 1.42(4) mu B.