Local atomic arrangement and electronic structure of nanocrystalline transition metal oxides determined by X-ray absorption spectroscopy

The local crystal structure and electronic configuration of transition metal in X-ray amorphous MnO₂ and CrO₂ nanocrystals have been examined by using X-ray absorption (XAS) spectroscopy at Mn K and Cr K-edges. The Mn K-edge XAS study reveals that tetravalent manganese ions are stabilized in α-MnO₂-type local atomic arrangement consisting of the intergrowth of edge- and corner-shared MnO₆ octahedra. On the other hand, it is found from Cr K-edge XAS results that nanocrystalline CrO₂ possesses two different kinds of local structures around chromium, that is, Cr₂O₃-type with octahedral site and CrO₃-type with tetrahedral site. The presence of Cr^+VI species on the surface would be helpful for Li grafting process, giving rise to excellent electrochemical performances. This work can be regarded as a strong evidence for the usefulness of XAS to study nanocrystalline electrode materials.