Elucidation of the Al/Si Ordering in Gehlenite Ca2Al2SiO7 by Combined 29Si and 27Al NMR Spectroscopy/Quantum Chemical Calculations

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• 作者：Pierre Florian ; Emmanuel Veron ; Timothy F. G. Green ; Jonathan R. Yates ; Dominique Massiot
• 刊名：Chemistry of Materials
• 出版年：2012
• 出版时间：November 13, 2012
• 年：2012
• 卷：24
• 期：21
• 页码：4068-4079
• 全文大小：604K
• 年卷期：v.24,no.21(November 13, 2012)
• ISSN：1520-5002

文摘

We have investigated the Al/Si ordering in the pseudoisolated pairs of tetrahedral sites of the structure of crystalline gehlenite Ca₂Al₂SiO₇ by means of ²⁹Si and ²⁷Al NMR and first-principles quantum mechanical calculations. ²⁹Si NMR spectra of isotopically enriched samples enables the precise determination of the population of the two silicon sites Si鈥?OAl)_3-n(OSi)_n (n = 0, 1) and hence the amount of Al鈥揙鈥揂l linkages. This leads to a reliable and model-free quantification of the departure from the Loewenstein rule and to an experimental Al/Si ordering enthalpy of 50.4 卤 1.6 kJ/mol fully reproduced by the quantum mechanical calculations. The seven aluminum sites arising from the Al/Si substitutions Al鈥?OAl)_4-p(OSi)_p (0 鈮?p 鈮?4) and Al鈥?OAl)_3-p(OSi)_p (p = 0, 1) are identified by ²⁷Al MAS, MQMAS, and {²⁹Si}²⁷Al HMQC experiments, with their quantification being consistent with a fully disordered arrangement of the tetrahedral pairs in the a鈥揵 plane of the structure. Assignments of those strongly overlapping lines are further confirmed by density functional theory (DFT) calculations performed on a series of 2 脳 25 supercells. An experimental and computational variation of 鈭? ppm of the ²⁷Al isotropic chemical shift is obtained for the substitution of one Al by one Si in the second coordination sphere of a central Al atom. ²⁹Si and ²⁷Al isotropic chemical shifts are seen to be sensitive primarily to short-range structural variations whereas a more complex behavior related to the nearby presence of Loewenstein-violating pairs is observed for the ²⁷Al quadrupolar coupling constant. Decomposition of the calculated EFG tensors into a sum of local, nonlocal, and ionic components demonstrates that it is almost entirely determined by the local electronic structure near the T₁ nucleus. The width of the distribution of NMR parameters is seen to strongly correlate to the degree of ordering present in the material. Scalar coupling constants ²J(T鈥揙鈥揟) (with T = Al, Si) are found to be linearly related to the TOT bond angle.

Keywords:

Al/Si ordering; aluminosilicate minerals; NMR spectroscopy; first-principle calculations; silicon-29; aluminum-27