Electronic structure and bonding of beta-SiAlON

The alpha- and beta-SiAlONs are ceramic solid solutions with charge-neutral substitutions in alpha- and beta-Si3N4. They have high potential for appli cations as structural materials. We have calculated the electronic structur e and bonding of beta-Si6-zAlzOzN8-z, for z = 0, 1, 2, 3, 4 using a first-p rinciples method. Total energy calculations show that the bulk modulus of b eta-Si6-zAlzOzN8-z decreases as z increases, in general agreement with expe rimental trends. Simultaneous substitution of the (Si,N) pair by (Al,O) res ults in impurity-like states in the upper portion of the bandgap of beta-Si 3N4. As z increases, more and more states are introduced into the gap, form ing a new conduction band (CB) edge for SiAlON. At z = 4, the calculated ba ndgap is similar to 1.3 eV. Density of states (DOS) calculations show the t op of the valence band remains steep for all z, and the bottom of the CB is formed predominately by Si-O antibonding states. Orbitally resolved partia l DOS calculations in the CB region are used to predict the trends of the e lectron-energy-loss near-edge spectra (ELNES) of Si-L-2.3, Al-L-2.3, Si-K, Al-K, O-K, and N-K edges in beta-SiAlON. The impurity-like states near the CB edge result in pre-edge structures in all ELNES spectra, Effective charg e and bond order calculations show that the overall bond strength in beta-S iAlON decreases only slightly as z increases. Although the stronger Si-N bo nds are replaced by weaker Al-O bonds, the remaining Si-N and Al-O bonds ac tually strengthen as z increases because of the effective charge redistribu tion after substitution. This is a very interesting finding that may partly explain the superior mechanical properties of the SiAlON system that rende r them suitable for structural applications.