Abstract
Results of SCF-Xα-SW molecular orbital calculations on (FeO4(OH)2)7− and (FeO6)9− clusters are used to investigate the differences between Fe-O and Fe-OH bonding in hydroxyl-bearing iron oxides and silicates. The Fe3+-OH− bond is more ionic, and has a smaller spinpolarization, then the Fe3+-O2− bond. The smaller spinpolarizability of OH− ligands explains why superexchange interactions between hydroxo-bridged Fe3+ cations are much weaker than those between oxo-bridged Fe3+ cations.
Replacement of oxygens in the Fe3+ coordination environment by OH− ligands appears to promote the covalency between Fe3+ centers and O2− oxygens. The increased covalency lowers the effective spin of the Fe atom. This, in turn, explains the decreased magnetic hyperfine fields at the Fe nucleus in FeOOH polymorphs relative to those found in Fe3+ oxides.
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Sherman, D.M. SCF-Xα-SW MO Study of Fe-O and Fe-OH chemical bonds; applications to the mössbauer spectra and magnetochemistry of hydroxyl-bearing Fe3+ oxides and silicates. Phys Chem Minerals 12, 311–314 (1985). https://doi.org/10.1007/BF00310345
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DOI: https://doi.org/10.1007/BF00310345