Superexchange interaction between lanthanide f1 ions. Spin-Hamiltonian calculations for the 90° and 180° f1-f1 superexchange

Abstract

Exchange interaction between two lanthanide or actinide ions of f1 configuration bridged by common diamagnetic ligands is theoretically studied using a modified version of the superexchange theory developed in this paper. Exchage spin Hamiltonians were calculated for the M2L10 and M2L11 dimers serving as models of the 90° and 180° f1-f1 superexchange, respectively. Spin-orbit coupling and crystal field splitting of the f1 configuration (resulting in the Γ7 ground Kramers doublet and the effective spin S = 1/2 of the metal ion), virtual transfers of electrons of the type 4f1(A)-4f1(B) → 4f0(A)-4f1(B)5d1(B) via ns(L) and np(L) valent orbitals of the bridging ligands, and exchange pathways in these dimers are considered in detail. The f1-f1 superexchange is found to be extremely anisotropic and very sensitive to the geometry of the dimer. The spin Hamiltonian of the M2L10 dimer is H = JxSAxSBx + JySAySBy + JzSAzSBz. where the exchange parameters are rationalized in terms of Jπσ and Jππ parameters referring, respectively, to the π-σ and π-π pathways of the 4f(A) → np(L) → 5d(B) electron transfers, Jx = 2Jπσ - Jππ, Jy = Jπσ + Jππ and Jz = -Jπσ + Jππ. The Jπσ and Jππ values are analytically expressed through (4f|np) and (5d|np) overlap integrals, orbital energies and intraionic Slater parameters. Exchange interaction between f1 ions in the M2L11 dimer is described by an antiferromagnetic Ising Hamiltonian H = |Jππ|SAzSBz, where the z axis connects two metal ions. Unusual magnetic properties of MUO3 (M = Li, Na, K and Rb) and Li3UO4 oxides involving U5+ (5f1) ions and BaPrO3 distorted perovskite are discussed in the light of these theoretical results.