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

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⁺-O^2? 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 O^2? 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.