ELECTRONIC-STRUCTURE OF TETRAHEDRAL IRON(III) SULFUR CLUSTERS IN ALKALINE THIOFERRATES - AN X-RAY-ABSORPTION STUDY

X-ray absorption spectra of Na5FeS4 and KFeS2 containing separate and interconnected edge-shared FeS45- tetrahedra, respectively, are reported and interpreted in terms of an S = 5/2 ground state for Fe3+ in both compounds. A rather small value of the ligand-to-metal charge transfer energy Delta(eff) was indicated by the comparison of the experimental Fe 2p XAS spectrum with atomic multiplet calculations, reflecting an almost 1 : 1 admixture of d(5) and d(6)L (L hole on the ligand) character in the ground state for Na5FeS4. The broadening of the main peak toward higher energies when going from Na5FeS4 to KFeS2 is attributed to symmetry lowering from T-d to D-2d. Ligand-field parameterization schemes, such as the angular overlap model, are found to be quantitatively not applicable for the systems under consideration. The phase diagram of d(5) ions and its dependence on the crystal-field splitting (10Dq) acid the ligand-to-metal charge transfer energy (Delta(eff)) are discussed and it is shown that an intermediate spin state (S = 3/2) can become stable for negative Delta(eff) and moderate values of 10Dq and metal-ligand hybridization. This presents another limit for the ligand-field approach and the Tanabe-Sugano diagrams, where only S = 5/2 and S = 1/2 ground states are predicted. For the Fe3+ in a tetrahedral field the theory indicates that it is unlikely that the S = 3/2 and S = 1/2 states become the ground state since the gain in energy due to ligand-ligand coupling and the relatively small charge transfer energy (both tending to lower energetically the S = 3/2 state) are not sufficient to overcome the high exchange stabilization of the high-spin (S = 5/2) state. (C) 1995 Academic Press, Inc.