Interpretation of Si-L2,3 edge electron energy loss near edge structures (ELNES) from intergranular glassy film of Si3N4 ceramics

First-principles molecular orbital calculations of model clusters for Si3N4, Si2N2O, SiO2 crystals are made by the discrete variational (DV)-X alpha method in order to interpret features that appear in the Si-La-2,La-3 edge ELNES spectra, They are understood in terms of interaction among molecular orbitals for their structural units, i.e., [SiX4](n-) tetrahedra (X = N, O), Two parameters, i.e., the N/(N + O) ratio, M-N, and the number of N atoms included in a unit tetrahedron, X-N, are found to determine the spectral features. On the basis of this knowledge, spatially resolved ELNES from intergranular glassy films of high-purity Si3N4-SiO2 ceramics is interpreted. The shift in the first peak by 0.9 eV can be ascribed to M-N = 0.43 +/- 0.06 in the intergranular film. The N content satisfactorily agrees with that determined by line-profiling of ELNES, Broadening of the first peak can also be explained consistently by considering this amount of N in the glassy film, An example model-cluster of the glassy film, [Si7N13O7](25-) is found sufficient to reproduce the ELNES from the glassy firm without inclusion of further complexity or inhomogeneity. The presence of N in the intergranular film is proposed to be the result of the topological requirement imposed by the absence of a broken bond, A model structure that may be useful for further structural optimization is given.