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

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