NEAR-EDGE STRUCTURE OF OXYGEN IN INORGANIC OXIDES : EFFECT OF LOCAL GEOMETRY AND CATION TYPE

NEXAFS measurements at the oxygen K-edge have been carried out on a variety of crystalline oxide samples to examine the effects of differences in local coordination environment of oxygen on near-edge structure. All spectra display an intense white line at about 543 eV and a strong shape resonance about 20 eV above the edge. The white line shifts to higher energies with increasing oxygen coordination number, although differences in nearest-neighbor type can affect this correlation. In addition, the Ca and transition-metal contain- ing oxides display features 6-10 eV below and 6-9 eV above the white line. The intensities of these features vary inversely with the number of 3d-electrons but their energies are independent of cation oxidation state. Large changes in coordination number and geometry affect edge structure less than variations in types of nearest- neighbor cations. The inverse distance-energy relationship is poorly obeyed in these compounds when oxygen- cation bond lengths are compared with the position of the strongest shape resonance for a given structure type. The near-edge structure is qualitatively interpreted using the results of recent Xa multiple scattered-wave calcu- lations.