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

Abstract

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 C_2v 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 AlF₃ materials.