Distribution of solute atoms in β- and spinel Si _6-zAl _zO _zN _8-z by Al K-edge x-ray absorption near-edge structure

Local environments of solutes in β- and spinel Si _6-zAl _zO _zN _8-z are investigated by means of Al K x-ray absorption near-edge structure. The experimental spectra are found to be the same throughout the wide solubility range. This suggests that the local environments of Al are independent of the solute concentration. First-principles band-structure calculations are systematically made to interpret the experimental spectra. Effect of a core hole was included into the calculation. Theoretical spectra were obtained using variety of different model structures constructed by a set of plane-wave pseudopotentials calculations in our previous study [K. Tatsumi, I. Tanaka, H. Adachi, and M. Yoshiya, Phys. Rev. B 66, 165210 (2002)]. The numbers of models were 51 and 45 for both β and spinel, respectively. They are classified and averaged according to the local atomic structure of Al solutes. The combination of experimental spectra and theoretical results can unambiguously lead to the conclusion that Al atoms are preferentially coordinated by O atoms in both β and spinel phases. This is consistent with the conclusion obtained by the first-principles total-energy calculations. In the spinel phase, Al atoms are found to be located preferentially at the octahedral cationic site. This agrees with the conclusion in a recent report on the nuclear magnetic resonance experiment.