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.

Non-aqueous sol–gel synthesis of nano-structured metal fluorides

“…This similarity indicates that disorder pre-formed in the precursor is the reason for the high Lewis acidity of HS-AlF 3 . The EXAFS spectra support the view that in going from crystalline AlF 3 over dry Al-F-gel to HS-AlF 3 there is a decrease in Al coordination number with HS-AlF 3 significantly differing from the others [40]. Indeed, five-fold coordinated Al can be stable as shown by the ab initio calculations for the surface of b-AlF 3 [39].…”

“…Although OR is replaced by the more electronegative F the electronic effect can be small assuming that not the voluminous OR but the smaller F ligand is shared between two Al centres. According to their EXAFS spectra aAlF 3 , b-AlF 3 , dry Al-F-gel, dry Al-F-gel heated under inert atmosphere, and HS-AlF 3 can be divided into two groups of rather similar behaviour [40]. The first group comprises a-AlF 3 , b-AlF 3 and the heated Al-F-gel with the spectra b-AlF 3 and the heated gel being very similar.…”

Spectroscopic characterization of crystalline AlF3 phases

“…An experimental and theoretical description of the characteristics of the F-K edge absorption X-ray structures (XANES) of a-, band an AlF 3 -phase, depicted as tetragonal AlF 3 (u), is given in [15,16]. XPS data of these three AlF 3 phases were reported by Boese et al [18,19] where charge referencing had been either done to the binding energy of C 1s or Au 4f 7/2 photopeaks.…”

Characterization of Lewis acid sites on the (100) surface of β-AlF3: Ab initio calculations of NH3 adsorption

“…The surface of β-AlF 3 is known to catalyse many fluorine and chlorine exchange reactions 5,6,7 . Experimental studies, including X-ray absorption near-edge structure at the F K-edge 8 , temperature programmed desorption of NH 3 9 and the temperature programmed dismutation reaction of CCl 2 F 2 10 , suggest that amorphous HS-AlF 3 has a structure closely related to β-AlF 3 . An understanding of the atomic scale structure and adsorption properties of the β-AlF 3 surface may therefore enable a better understanding of the catalytic nature of both β-AlF 3 and HS-AlF 3 .…”