The local crystal structure and electronic band gap of β-sialons

Abstract

The electronic properties of β-Si3N4 and β-sialons (β-Si6−z Al z O z N8−z ) solid solutions were characterized using a combination of X-ray emission spectroscopy (XES), X-ray absorption spectroscopy (XAS), and density functional theory (DFT). The electronic structure measurements reveal a single bonding environment for both the Si and Al atoms, which corresponds to a specific nonrandom structural arrangement of the Al–O solute atoms into nanotube-like clusters or channels, running parallel to the c-axis of the β-Si3N4 host structure. Compared to an arrangement of alternating Si–N and Al–O slabs (“Dupree model”), lower total energy and overall better agreement to the experimentally observed electronic features confirm this “Al–O nanotube” model for β-sialon originally proposed by Okatov to be closer to the true chemical topology of the β-Si6−z Al z O z N8−z solid solution series. The β-sialons are shown to be wide band gap semiconductors with the band gap reduction arising from the O p-states moving toward the Fermi level. This band gap reduction provides the ability for direct band gap transitions, which is very important for practical applications. In contrast to the previous observations, both measurement and theory indicate a linear dependence of band gap energy with composition z. The experimental (theoretical) electronic band gaps of β-Si6−z Al z O z N8−z with z = 0.0, 2.0, and 4.0 as determined by XAS/XES (DFT) are 7.2 ± 0.2 (5.88), 6.2 ± 0.2 (3.45), and 5.0 ± 0.2 (2.39) eV, respectively. The considerable discrepancy between experimental and theoretical values is attributable to the shortcomings of DFT, which often underestimates the electronic band gap energy.