First-order phase transition in UO2: U-235 and O-17 NMR study - art. no. 104404

A nuclear magnetic resonance (NMR) study is reported for a 5f-cooperative J ahn-Teller (JT) system with antiferromagnetic ordering, UO2, where we have performed magnetic actinide (U-235) as well as nonmagnetic ligand (O-17) NM R studies in a SS-electron system. The observed U-235 hyperfine interaction is consistent with an axially symmetric Sf-wave-function character for U4 below a first-order transition temperature T-N = 30.8 K Thus, the orbital degeneracy in the cubic crystalline field is lifted by a magnetic ordering combined with a JT distortion. On the other hand, modulation phenomena in t he O-17 spin-echo decay provide evidence of a lattice distortion just below T-N, which gives rise to an axially symmetric electric-field gradient at t he oxygen sites. These results indicate that, in a cooperative JT transitio n which occurs as a result of coupling between 5f quadrupoles and lattice d istortion, the behavior of the 5f quadrupoles can be investigated using ura nium NMR, white the lattice distortion can be studied with the nonmagnetic Ligand NMR. Among magnetic structures and JT distortions, proposed on the b asis of neutron scattering experiments and theoretical work, the noncolline ar 3k-type structure is the most likely, because either the 1k or 2k would cause an orthorhombic distortion. With respect to low-energy spin-wave exci tations, nuclear spin-lattice relaxation rates T-1(-1) at both sites shaw a T-7 behavior below T-N, which suggests the presence of gapless excitations due to strong magnon-phonon coupling.