The electronic structure of trigonal fivefold-coordinated Fe(III) molecules has been studied. The ligand-field model that we use takes into account the covalency effect which modifies the ligand-field parameters. The complete matrices for electron-electron repulsion, ligand-field, and spin-orbit coupling, have been diagonalized, including the Zeeman interaction in the case of magnetic-property calculations. The energy levels position can be expressed as a function of the ratio Δ/ζ, Δ being the energy gap ${}_{}{}^6{}_{}{}^{}$${\mathit{A}}_{1\mathrm{-}}^4$E, and ζ the spin-orbit-coupling parameter. It is shown that, by varying this ratio, the intermediate spin level ${}_{}{}^4{}_{}{}^{}$E${(}^4$${\mathit{T}}_1$) can be stabilized as the ground state, instead of the ${}_{}{}^4{}_{}{}^{}$${\mathit{A}}_2$${(}^4$${\mathit{T}}_1$) level in the case of a tetragonal distortion. Near the crossover point ${(}^6$${\mathit{A}}_{1\mathrm{-}}^4$E transition), strong quantum admixtures and large zero-field splittings occur: the ground Kramers doublet contains 41% high-spin state and 59% intermediate-spin state; the corresponding ground-state wave functions are associated with ${\mathit{M}}_{\mathit{J}}$=±5/2; the first-excited low-lying Kramers doublet associated with ${\mathit{M}}_{\mathit{J}}$=±3/2 is located at 102.5 ${\mathrm{cm}}^{\mathrm{-}1}$. Characteristic temperature dependences of the effective magnetic moment are calculated as a function of Δ/ζ. Taking into account the effect of the temperature on the considered parameters, the magnetic moment can vary more or less sharply from a high-spin value to an intermediate-spin value.

• Received 24 March 1993

DOI:https://doi.org/10.1103/PhysRevB.48.6676