X-ray Absorption Near-Edge Structure of Grain-Boundary-Segregated Y and Zr in Creep-Resistant Alumina

X-ray absorption near-edge structure (XANES) depends on stereochemical features of coordination polyhedra around probe atoms. K-edge XANES of Y and Zr segregated in alumina grain boundaries has been obtained and analyzed using metallic Y and Zr, Y2O3, YAG, and monoclinic ZrO2 as standards. Grain-boundary-segregated Y and Zr show a positive chemical shift, and the magnitude of the shift, as compared with that of Y2O3 and ZrO2, respectively, is different for Y and Zr, indicating that, relative to Y2O3 and ZrO2, charge transfers for the grain-boundary-segregated Y and Zr are different. This result is also supported by the strength of the threshold resonance. A pre-edge shoulder is seen in K-edge XANES for grain-boundary-segregated Y and Zr but not for Y2O3 and ZrO2. This shoulder is attributed to the 1s 3 4d transition, which is normally forbidden, but can occur because of d–p mixing, which is favored by a tetrahedral coordination configuration. These results suggest that some of the grainboundary-segregated Y and Zr have coordination configurations with a well-defined tetrahedral symmetry. The XANES results are compared with those obtained from EXAFS. Implications of these results for understanding of the enhanced creep resistance in alumina are also discussed.