Ralph Schenker, Stephan Heer, Hans U. Güdel and Høgni Weihe
Optical Dimer Excitations and Exchange Parameters in (Et₄N)₃Cr₂F₉: First Observation of the ⁴A₂ ²A₁ Transition
Inorg. Chem. 40, 1482-1488 (2001)                 

Abstract: The synthesis, crystal growth, and polarized optical absorption spectra in the visible and near-UV of (Et₄N)₃Cr₂F₉ are reported. In the energy range 25800-27700 cm^–1 the ⁴A₂ ²A₁ (O notation) ligand field transition can be resolved in detail for the first time in any Cr³⁺ compound. This allows the determination of the antiferromagnetic ground-state exchange splitting with great accuracy: J = 25.9 cm^–1 and j = 0.27 cm^–1 using the Hamiltonian H = J(S_A·S_B) - j(S_A·S_B)², where j leads to deviations from the regular Landé pattern. The temperature dependence of the magnetic susceptibility is nicely reproduced by these parameters. A comparison with Cs₃Cr₂Cl₉ and Cs₃Cr₂Br₉ reveals an exponential dependence of the ground-state splitting upon the Cr-Cr distance in the [Cr₂X₉]^3- dimers. This is the result of a dominant s-type orbital exchange pathway along the Cr-Cr axis.