X-ray Absorption Spectroscopy for Estimation of Oxidation State, Chemical Fraction and Local Atomic Structure of Materials

This work discussed the role of X-ray absorption spectroscopy (XAS) in determining the oxidation state, chemical fraction, and local atomic structure of the materials. These aspects of XAS were discussed by taking LiNiO2 and Mn3O4 as prototype materials. The oxidation state of metal ions in these oxides was estimated with the help of XAS spectra of the reference oxides such as NiO (in the case of LiNiO2), MnO, Mn2O3, and MnO2 (in the case of Mn3O4). Analysis of the oxidation state was performed from the main absorption edge which was estimated from half of the step height. This showed that the Ni K-edge absorption edge of LiNiO2 is slightly above that of NiO. In the case of Mn ions, the main absorption edges show a linear variation with the oxidation states. This estimates the presence of a mixed oxidation state (2.6+) of Mn ions in Mn3O4. Linear combination fitting results exhibit that almost 35% of ions are in a 2+ oxidation state. The remaining ions are in a 3+ oxidation state. Thus, XAS can determine the fractions of each oxidation state of a particular ion in a given material. Quantitative information on coordination number and bond distance of nearest neighbor for a given element of a material is another important use of this technique.