Neutron scattering investigation of magnetic phase transitions and crystal-field splitting in UX3 (X ≡ Br, Cl)

Neutron scattering experiments were performed in the temperature range from 1.5 to 293 K on polycrystalline samples of the non-metallic 5f3 electron systems UBr3 and UCl3. The hexagonal crystal structure P63/m was refined at T = 293 and 10K. UBr3 orders at TN = 5.4(1) K with a second-order transition to a 0+-configuration of magnetic moments of magnitude μU(3.9K) =2.08(2)μB oriented parallel to the c axis. At Tt1 =3.0(5) K occurs a second transition to an ordered antiferromagnetic ground state, corresponding to the lattice constants am = √ 3a, bm = a, cm = 2a. This structure is anticentered in the basal plane. The magnetic moments of magnitude μU(1.6K) 0.82(4)μB are oriented parallel to the b axis. In both magnetic phases the antitranslation (0, 0, 1/2) suggests predominantly antiferromagnetic exchange within the chains of uranium ions along the c axis. The situation is similar for UCl3: the first transition at TN = 6.5(1) K with an ordered magnetic moment μU(4.0K) of 1.8(1)μB, and the second transition at Tt = 3.8(5) K with a magnetic moment μU(1.6K) of 0.8(1)μB. By means of inelastic neutron scattering we have measured the crystal-field splitting of UBr3 and UCl3 and determined the crystal-field parameters. The assumption of anisotropic magnetic interactions reasonably predicts the value of the zero-field magnetisation and provides an explanation for the magnetic phase transitions. (author)