Abstract
The spatial ordering of aluminum atoms in CsAl(SiO~3~)~2~ and 3Al~2~O~3~·2SiO~2~ was probed by ^27^Al dipolar solid‐state NMR spectroscopy. The ^27^Al response to a Hahn spin‐echo pulse sequence in a series of aluminum‐containing model crystalline compounds demonstrates that quantitative ^27^Al homonuclear dipolar second moments can be obtained to within ±20% of the theoretical values, if evaluation of the spin‐echo response curve is limited to short evolution periods (2__t__~1~ ≤ 0.10 ms). Additionally, selective excitation of the central transition $m = {1 \over 2} \to - {1 \over 2}$ is necessary in order to ensure quantitative results. Restriction of spin exchange affecting the dephasing of the magnetization may decelerate the spin‐echo decay at longer evolution periods. Considering these restraints, the method was used to probe the spatial distribution of aluminum atoms among the tetrahedral sites in two aluminosilicate materials. Experimental ^27^Al spin‐echo response data for the aluminosilicates CsAl(SiO~3~)~2~ (synthetic pollucite) and 3Al~2~O~3~·2SiO~2~ (mullite) are compared with theoretical data based on (I) various degrees of aluminum–oxygen–aluminum bond formation among tetrahedrally coordinated aluminum atoms (Al—O—Al) and (II) the maximum avoidance of Al—O—Al bonding. Analysis of the second moment values and resulting echo decay responses suggests that partial suppression of spin exchange among aluminum atoms in crystallographically distinct sites may contribute to the ^27^Al spin echo decay in 3Al~2~O~3~·2SiO~2~, thus complicating quantitative analysis of the data. Silicon‐29 and aluminum‐27 magic angle spinning (MAS) NMR spectra of 3Al~2~O~3~·2SiO~2~ are consistent with those previously reported. The experimental ^27^Al spin‐echo response behavior of CsAl(SiO~3~)~2~ differs from the theoretical response behavior based on the maximum avoidance of Al—O—Al bonding between tetrahedral aluminum sites in CsAl(SiO~3~)~2~. A single unresolved resonance is observed in both the silicon‐29 and aluminum‐27 MAS spectra of CsAl(SiO~3~)~2~. Copyright © 2003 John Wiley & Sons, Ltd.