F K-edge X-ray absorption near-edge structure (XANES) of AlF3 polymorphs: Combining ab initio calculations with Walsh correlation diagrams

The X-ray absorption near-edge structures (XANES) at the F K-edge of α-AlF3, β-AlF3 and a tetragonal AlF3 phase are analysed by a combination of ab initio calculations with the FEFF8 code and a phenomenological discussion of local molecular orbital (MO) symmetries at the absorbing fluorine atoms. By means of a Walsh correlation diagram it is shown that the two intense absorption bands observed at the F K-edges of the AlF3 polymorphs can be interpreted as transitions to anti-bonding MOs in [Al-F-Al]-units that have C2v and D ∞h point group symmetries. The energies of both anti-bonding orbitals are very insensitive to the angle between the Al-F bonds, which explains the close similarity of the XANES signatures from the three polymorphs. The FEFF8 analysis shows that the increased broadening of the XANES structure for β-AlF3 and the tetragonal AlF3 phase is due to the superposition of the individual absorption spectra from the crystallographically distinct F species. The interpretation in terms of local MOs provides for the first time a "chemically intuitive" approach to investigations of solid fluorides by XANES spectroscopy and provides a simple conceptual framework for the discussion of the electronic structure in AlF 3 materials. © the Owner Societies 2006.