Local structure of β′-sialons: an EXAFS study study

The local environments of aluminium and silicon in β′-sialons, Si6−zAlzN8−zOz, have been studied by extended X-ray absorption fine structure spectroscopy. The silicon and aluminium atoms are always in tetrahedral coordination and the refined average bond length between two atomic positions becomes a measure of the distribution of nitrogen and oxygen over the nearest neighbour positions to the absorbing atom. Two β′-sialons samples with different composition, z=1 and z=2.7, were used in the study with α-Si3N4, SiO2 (cristobalite, albite and a xerogel), zeolite NaA, anorthite and AIN as reference substances. The calculated distances to the first coordination sphere for silicon were 0.169 nm for z=1 and 0.168 nm for z=2.7, compared to 0.171 nm for Si3N4 under the same conditions. In the aluminium environment, the calculated distances were 0.177 nm for z=1 and 0.176 nm for z=2.7. The results indicate that the β′-sialons are built up by aluminium- and silicon-containing tetrahedra with mixed oxygen and nitrogen environments, although there seems to be a stronger tendency for the formation of Si-N and Al-O bonds, compared to Si-O and Al-N bonds. The results are not precise enough to allow calculation of the contribution from different bond types to the observed average bond lengths; in particular this is due to the short (< 200 eV) AlK edge available for structural studies of the β′-sialons.