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

We have investigated the Al/Si ordering in the pseudoisolated pairs of tetrahedral sites of the structure of crystalline gehlenite Ca 2 Al 2 SiO 7 by means of 29 Si and 27 Al NMR and first-principles quantum mechanical calculations. 29 Si NMR spectra of isotopically enriched samples enables the precise determination of the population of the two silicon sites Si–(OAl) 3- n (OSi) n ( n = 0, 1) and hence the amount of Al–O–Al 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 ± 1.6 kJ/mol fully reproduced by the quantum mechanical calculations. The seven aluminum sites arising from the Al/Si substitutions Al–(OAl) 4- p (OSi) p (0 ≤ p ≤ 4) and Al–(OAl) 3- p (OSi) p ( p = 0, 1) are identified by 27 Al MAS, MQMAS, and { 29 Si} 27 Al HMQC experiments, with their quantification being consistent with a fully disordered arrangement of the tetrahedral pairs in the a–b 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 × 25 supercells. An experimental and computational variation of −3 ppm of the 27 Al isotropic chemical shift is obtained for the substitution of one Al by one Si in the second coordination sphere of a central Al atom. 29 Si and 27 Al 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 27 Al 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 T 1 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 2 J (T–O–T) (with T = Al, Si) are found to be linearly related to the ∠TOT bond angle.