Beschreibung:
<jats:p>Noncrystalline aluminum silicate precursors MP1 and MP3 with different ratios of Al in tetrahedral (<jats:sup>[4]</jats:sup>A1), pentahedral (<jats:sup>[5]</jats:sup>A1), and octahedral (<jats:sup>[6]</jats:sup>A1) position were prepared from aluminum sec‐butylate and tetraethoxysilane (Al/Si ratio: 3/1) making use of a slow and rapid hydrolysis process, respectively. The MP2 precursor was synthesized from aluminum sec‐butylate and silicon tetrachloride (Al/Si ratio: 4/1) by rapid hydrolysis. All aluminum silicate gels were heat‐treated to temperatures just prior to crystallization to mullite and γ‐Al<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub>, respectively (MP1 and MP3: 800°C; MP2: 300°C). Al K near‐edge absorption spectra (XANES) of the mullite precursors and of several suitable crystalline reference materials were measured using monochromatic synchrotron radiation. The reference XANES spectra yielded a linear correlation between the energy position of the first inflection point of the white lines and the frequency of sixfold‐coordinated Al (<jats:sup>[6]</jats:sup>A1) in the samples. A similar though less straightforward correlation seems to hold also for the white line intensities. From these findings, the actual <jats:sup>[61]</jats:sup>A1 frequencies for the mullite precursors under investigation were derived to be <10% (MP1), =30% (MP3), and =60% (MP2). The <jats:sup>[6]</jats:sup>A1 frequency distribution in the mullite precursors displays a trend similar to that determined by <jats:sup>27</jats:sup>A1 NMR spectroscopy.</jats:p>