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

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. Exchange spin Hamiltonians were calculated for the M2L10 and M2L11 dimers serving as models of the 90 deg. and 180 deg. 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 vertical bar np> and <5d vertical bar np>overlap integrals, orbital energies and intra-ionic Slater parameters. Exchange interaction between f1 ions in the M2L11 dimer is described by an antiferromagnetic Ising Hamiltonian H = vertical bar Jππ vertical bar 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. (author)