Chemical bonding and electronic structures of the Al₂SiO₅ polymorphs, andalusite, sillimanite, and kyanite: X-ray photoelectron- and electron energy loss spectroscopy studies

Abstract

We have undertaken a detailed analysis of the X-ray photoelectron spectra obtained from the three polymorphs of Al₂SiO₅; andalusite, sillimanite, and kyanite. Comparison of the spectra was made based on the chemical bonding and structural differences in the Al- and Si-coordination within each polymorph. The spectra for Si(2p) for all three polymorphs are nearly identical, consistent with the fact that all the Si atoms are in 4-fold (tetrahedral) coordination, whereas the binding energies, peak shapes, and peak widths for Al(2p) vary depending on the type of polymorph. The upper-valence band for all three polymorphs is characterized by four main features derived from O(2p), Al(3s), Al(2p), Si(3s), and Si(3p), and the differences in their contributions are observed. The density of state of the Al₂SiO₅ polymorphs is relatively featureless compared to those observed from α-SiO₂ and α-Al₂O₃, suggesting that the orbital overlaps span a greater range in energy. The observed band gap energy for Al₂SiO₅ (sillimanite) was ~9.1eV, a value in between those for α-SiO₂ (~8.6eV) and α-Al₂O₃ (~9.6eV). The conduction band feature of Al₂SiO₅ was experimentally compared to those of α-SiO₂ and α-Al₂O₃, and shown that it is indeed intermediate between the α-SiO₂ and α-Al₂O₃ phases.