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

Abstract

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–(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 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–b plane of the structure. Assignments of those strongly overlapping...