Schroeder Sven L M, Weiher Norbert
Molecular Materials Centre, The University of Manchester, Sackville Street, PO Box 88, Manchester M60 1QD, UK.
Phys Chem Chem Phys. 2006 Apr 21;8(15):1807-11. doi: 10.1039/b518124k. Epub 2006 Mar 10.
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.
通过使用FEFF8代码进行从头算计算，并对吸收氟原子处的局域分子轨道（MO）对称性进行唯象讨论，分析了α-AlF₃、β-AlF₃和四方相AlF₃在F K边的X射线吸收近边结构（XANES）。借助沃尔什相关图表明，在AlF₃多晶型物的F K边观察到的两个强吸收带可解释为向具有C₂ᵥ和D∞ₕ点群对称性的[Al-F-Al]单元中的反键MO的跃迁。两个反键轨道的能量对Al-F键之间的夹角非常不敏感，这解释了三种多晶型物的XANES特征非常相似的原因。FEFF8分析表明，β-AlF₃和四方相AlF₃的XANES结构展宽增加是由于晶体学上不同的F物种的各个吸收光谱的叠加。基于局域MO的解释首次为通过XANES光谱研究固体氟化物提供了一种“化学直观”的方法，并为讨论AlF₃材料中的电子结构提供了一个简单的概念框架。
