First-order Phase Transition in UO2: The Interplay of the 5f2–5f2 Superexchange Interaction and Jahn–Teller Effect

Abstract

The competition of the superexchange interaction and the cooperative Jahn–Teller (JT) effect in the first-order magnetic and structural phase transition in UO2 is analyzed. The effective spin Hamiltonian of the superexchange interaction between the neighboring U4+ ions in the cubic crystal lattice of UO2 is calculated for the first time in terms of a specially adapted kinetic exchange model. The 5f2–5f2 superexchange interaction is shown to be essentially non-Heisenberg: the effective spin Hamiltonian is anisotropic and contains large biquadratic terms, which formally correspond to a quadrupole–quadrupole interaction. The strength of the JT effect in UO2 is estimated from the calculations of the linear vibronic coupling constants between the local eg, t2g(1) and t2g(2) JT modes and the Γ5 ground state of U4+ performed in terms of a semiempirical crystal field model for 5f electrons. Despite the Γ5⊗eg and Γ5⊗t2g(2) parameters are found to be much larger than Γ5⊗t2g(1), the actual lattice distortion in UO2 is of a pure t2g(1) character. Our results indicate that the superexchange interaction is the major driving force of the phase transition in UO2, which causes the magnetic and orbital ordering and suppresses the cooperative JT effect.