Crystal and magnetic structure of the uranium digermanide UGe2

The crystal structure of the uranium digermanide UGe2 was determined and refined from single crystal X-ray diffraction data to R = 0.040, Rw = 0.052. UGe2 crystallizes with the orthorhombic ZrGa2 type (Cmmm) instead of the ZrSi2 type (Cmcm) as was previously assumed on the basis of rather old and unreliable results. Our magnetic study of a polycrystalline are-melted UGe2 sample confirms the onset of ferromagnetic interactions below ca. 52 K. Neutron powder diffraction studies show that the structure is collinear ferromagnetic, with uranium magnetic moments 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.