Magnetite, maghemite, and hematite have been the subject of numerous studies using vibration spectroscopy to determine their infrared- and Raman-active phonons. However, no complete and unambiguous set of experimentally observed optically active phonons has yet been reported for these iron oxides. The use of atomistic simulation methods with a transferable Buckingham potential provides new data for the phonon densities of states of magnetite and the two associated phases, hematite and maghemite.

The lattice vibrational modes of spinel-structured lithium manganese oxides have been calculated using atomistic modeling methods. The simulations allow the Raman and infrared spectra of lithiated, fully delithiated, and partially delithiated phases to be assigned for the first time. Calculations for the spinels LiMn2O4, λ-MnO2, and Li0.5Mn2O4 are compared with experimental Raman data measured for thin films of the oxides coated on a platinum electrode. The appearance of a number of new bands in the Raman spectrum of LiMn2O4 following partial extraction of lithium is shown to result from local lowering of the symmetry and Raman activation of modes which are optically inactive or only infrared active in LiMn2O4. The results support a model for the Li0.5Mn2O4 lattice in which the lithium ions are ordered. The deformation vibrations of lattice hydroxyl “defects” in λ-MnO2 have also been calculated; comparison of the calculated and experimental vibrational data supports a model in which hydroxyl species are localized at octahedral Mn vacancies.

The local structural modifications resulting from chemical extraction and reinsertion of lithium in spinels of composition LiMn2O4 and Li4/3Mn5/3O4 have been investigated by X-ray absorption spectroscopy (XAS) at the manganese K-edge. For LiMn2O4, Mn−O and Mn−Mn distances determined from the extended X-ray absorption fine structure (EXAFS) decrease significantly when lithium is extracted in 0.1 M HCl, but are restored to their original values when ∼90% of the lithium content is reconstituted by sorption in LiOH solution. The Mn−O and Mn−Mn distances for Li4/3Mn5/3O4 are shorter than for LiMn2O4, consistent with a higher manganese oxidation state and lower unit cell parameter for this compound, and show much smaller variations on lithium extraction and reinsertion. In addition lithium extraction from LiMn2O4 results in an increase in local structural order, followed by a return to the lower symmetry state of the parent material when lithium is reinserted. These changes are not observed in the spectra of Li4/3Mn5/3O4 and its delithiated or relithiated products, which show rather the effects of local structural perturbations due to manganese vacancies in octahedral sites. Evolution of the X-ray absorption near-edge structure (XANES), in particular the position of the main edge discontinuity and modifications in the pre-edge spectral structure, supports chemical analyses in showing that lithium extraction from LiMn2O4 produces a MnIV-type spinel oxide from a mixed MnIII/MnIV parent material and that reinsertion of lithium results in a return to a mixed oxidation state. By contrast, the XANES data for Li4/3Mn5/3O4 show that here the dominant manganese state is MnIV and that lithium extraction and reinsertion entail only trivial local electronic changes. The XAS results are therefore compatible with a mainly redox mechanism for lithium extraction and reinsertion for LiMn2O4, but a predominantly ion-exchange process in the case of Li4/3Mn5/3O4, with the insertion of charge-compensating protons into the lattice.

Protonated forms of the spinel manganese dioxide phase A-MnCb have been prepared by the ion-exchange of three different lithium manganate precursors. The mechanisms of lithium extraction and proton insertion were examined in each case by X-ray diffraction and chemical and thermal analyses. The amount of protons inserted by ion exchange differed according to the composition of the precursor lithium manganate, the distribution of cations between 8a tetrahedral and 16d octahedral sites, and the oxidation state of the manganese, all of which depend on the method of preparation. The proton sites in the A-MnOa materials were characterized using a combination of inelastic neutron scattering (INS) and infrared spectroscopies. The strongest features in the INS spectra, at 910 and 1080 cm™1, are assigned to OH deformation modes. These results are discussed in relation to thermal analysis and infrared data for the manganese dioxides pyrolusite /3-2) and synthetic ramsdellite, and the proton sites explained using a conventional model for the manganese oxide lattice. The 910 cm™1 mode dominated the INS spectra of all three A-MnOa materials and is assigned to lattice hydroxyl groups associated with the vacant 8a tetrahedral sites in the structure. The presence of an INS mode at 1080 cm™1 was observed for only one of the A-MnOa samples and is attributed to protons in interstitial sites, associated with a small amount of Mnm in the material. Variable temperature infrared spectroscopy and X-ray diffraction showed that the loss of water above 100 °C results in destruction of the A-MnOa spinel lattice and suggests that water plays an important role in stabilizing the protonated A-MnOa structure. A model is proposed in which protons associated with oxygen atoms at 16d octahedral vacancies form lattice water species.

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6Li and 7Li MAS NMR spectroscopy is used t o identify distinct lithium sites in t w o paramagnetic manganese oxides: spinel-type LiMn204 and rock salt-type Li2Mn03.

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