Isotropic magnetic exchange between anisotropic Yb(III) Ions. Study of Cs₃Yb₂Cl₉ and Cs₃Yb ₂Br₉ crystals

Despite the prevalent belief about a strong anisotropy of the magnetic exchange in rare-earth compounds, Cs_3-Yb₂Cl₉ and Cs₃Yb₂Br₉ crystals are found to exhibit fully isotropic exchange coupling between Yb³⁺ tons. In this article, we attempt to reveal the physical origin of this surprising feature. Our theoretical consideration is based on a model of the kinetic exchange between two octahedrally coordinated Yb³⁺ ions in their ground Kramers doublet states. It is shown that a mechanism of kinetic exchange involving intercenter electron hopping between 4f orbitals of two Yb³⁺ ions in a face-shared binuclear unit results in fully Isotropie antiferromagnetic exchange coupling, while a mechanism in which the electron jumps from the 4f to the 5d orbital gives rise to a highly anisotropic interaction. Comparison of these results with the experimental data along with qualitative arguments regarding the relative significance of these two contributions to the overall exchange indicate that, in face-shared Yb³⁺ binuclear units, the 4f ↔ 4f mechanism plays a dominant role.