Electronic Structures of Iron and Manganese Oxides with Applications to Their Mineralogy

The electronic structures of iron and manganese oxide and related minerals are investigated using Self-Consistent Field Xa Scattered Wave molecular orbital calculations on finite atomic clusters. Calculations on Fe0 6, FeO4, MnO6 and Fe04(OH)2 coordination polyhedra are used to investigate the nature of chemical bonds, electronic spectra, magnetism, and mechanisms of electron transport in these minerals. Agreement between observed and predicted electronic spectra, geochemical behavior, and physical properties shows that much of the electronic structures of transition metal oxides can be understood in terms of a localized electron description. The highly-delocalized orbitals arising from the Fe or Mn 4s and 4p atomic orbitals, however, are less reliably described by clusters of this size. Electronic spectra arising from the intraconfigurational d-d (ligand field) transitions and ligand to metal charge-transfer (LMCT) transitions were investigated by calculating one-electron orbital transition energies and deriving the relations between these energies and those of the spectroscopic multiplets. Experimental spectra in the near IR to near UV of several FeOOH and Fe203 minerals are obtained and compared with the results of theoretical calculations. The results give a consistent band assignment scheme and confirm the accuracy of the electronic structure calculations. The energies of LMCT transitions are of particular interest insofar as these transitions give rise to photochemical reactions between Fe-Mn oxides and organic molecules in natural waters. The calculations show that LMCT transitions in iron(III) and manganese(IV) oxides occur well into the near-ultraviolet. The lowest energy LMCT transition of manganese(IV) oxides is at 4.3 eV. The lowest energy LMCT transition in iron(III) oxides occurs at 4.7 eV. The strong optical absorption in the visible region observed in the spectra of iron(III) oxides results from ligand field transitions which are intensified by the magnetic coupling of next-nearest neighbor Fe atoms in the crystal structure. An additional electronic transition corresponding to the simultaneous excitation two magnetically-coupled Fe3+ cations is found at 480-530 nm in the spectra of the Fe203 and FeOOH minerals. The electronic structure calculations show that the ligand field theory parameter 1ODq for octahedrally coordinated Fe and Mn cations increases in the order Mn2+ < Fe2+ < Fe3+ < Mn3+ < Mn4+. Calculated values for 10Dq are in very good agreement with those obtained from experimental spectra. The molecular orbital calculations show that chemical bonds in iron and manganese oxides are not ionic and can have appreciable covalency depending on the formal oxidation state of the transition metal cation. The covalency of the metal-oxygen bond in octahedral coordination olyhedra is found to increase in the order Mn2+ < Fe2+ < Fe3+ < < Mn +. This trend in covalency among the different clusters is consistent with the observed geochemical behavior of iron and manganese. The calculated values for the ligand field theory parameter 1ODq and the energy of the lowest LMCT transition parallel the trend in covalency and show how electronic spectra can give information on chemical bonding in minerals. The spin-unrestricted calculations show that covalency reduces the electronic spin (magnetic moment) of the Fe or Mn cations and induce a net spin on the oxygen ligands. The relative decrease in the Fe3+ magnetic moment in the different Fe3+ coordination polyhedra agree with both neutron diffraction studies and trends in the magnetic hyperfine field at the Fe nuecleus measured by Mossbauer spectroscopy. The spin-transfer by covalency (spin-polarization of the ligands) results in the superexchange interaction responsible for antiferromagnetism. Electronic structure calculations on a (Fe0 4(OH)2)cluster show that superexchange between hydroxyl-bridged Fe 3+ cations will be weaker than that between oxo-bridged Fe3+ cations. The nature of intervalence charge-transfer and electron delocalization in mixed valence iron oxides and silicates is investigated from a calculation of the electronic structure of a mixed-valence (Fe2010)15dimer cluster. Here, the a-spin Fe2+ t2g electron is found to be localized to its parent cation center which, in turn, implies that electron delocalization in mixed-valence minerals occurs through the motion of small polarons (hopping of localized electrons) and not by the formation of metal-metal bonds. Approximate energies of optical intervalence charge transfer energies are in reasonable a reement with experiment and are found to result from a Fe2+(t2ga) + Fe3+(eg ) electronic excitation. Thesis Supervisor: Roger G. Burns Title: Professor of Mineralogy and Geochemistry