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 alpha-MnO2-type local atomic arrangement consisting of the intergrowth of edge- and comer-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.