Temperature dependences of rhombohedral lattice distortion and of ferromagnetic uranium ordering in the uranium monochalcogenides

By means of high-resolution neutron diffraction, high-quality powder samples of UX (X = S, Se, Te) were investigated in the temperature range from 8 to 299 K in order to establish the relation between ferromagnetic U ordering and rhombohedral lattice distortion as a function of temperature. For US, USe and UTe the rhombohedral angle α of the pseudocubic unit cell is at 10 K approximately 0.4°, 0.5° and 0.3° smaller than 90°, respectively. Within error limits, the determined ordered magnetic moment values of US and UTe in zero external magnetic field agree with previous neutron single crystal studies, but the larger value derived in the present study for USe, suggest that a higher degree of localisation than previously assumed should be considered for this compound.