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

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 alpha-SiO2 and alpha-Al2O3, suggesting that the orbital overlaps span a greater range in energy . The observed band gap energy for Al2SiO5 (sillimanite) was similar to 9.1 eV, a value in between those for alpha-SiO2 (similar to 8.6eV) and alpha-Al2O3 (similar to 9.6eV). The conduction band feature of Al2SiO5 was experimentally compared to those of alpha-SiO2 and alpha-Al2O3, and shown that it is indeed intermediate between the alpha-SiO2 and alpha-Al2O3 phases.