title: 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 alpha-AlF(3), beta-AlF(3) and a tetragonal AlF(3) 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 AlF(3) polymorphs can be interpreted as transitions to anti-bonding MOs in [Al-F-Al]-units that have C(2v) and D(infinity 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 beta-AlF(3) and the tetragonal AlF(3) 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.
authors.affiliation: Molecular Materials Centre, The University of Manchester, Sackville Street, PO Box 88, Manchester M60 1QD, UK. s.schroeder@manchester.ac.uk
authors.forename: Sven L M
authors.initials: SL
authors.lastname: Schroeder
authors.forename: Norbert
authors.initials: N
authors.lastname: Weiher
mesh_terms: D000465:Algorithms; D017607:Aluminum Compounds / Q000737:chemistry*; D005459:Fluorides / Q000737:chemistry*; D007477:Ions; D046911:Macromolecular Substances / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D011789:Quantum Theory; D013057:Spectrum Analysis; D014965:X-Rays
chemical_list: D017607:Aluminum Compounds; D007477:Ions; D046911:Macromolecular Substances; D005459:Fluorides; C032311:aluminum fluoride