3QMAS of three aluminum polycations: space group consistency between NMR and XRD

Three aluminum polycations, Al-13 is an element of -Keggin Johansson-type, Al-13 Mogel-type and Al-30 delta -Taulelle-type have been measured by Al-27 solid State NMR, using MAS and 3QMAS spectroscopies. The number of inequiv alent aluminum obtained by these high resolution NMR experiments leads to a comparison with their respective XRD results obtained on single crystal. I n all three cases, the inequivalent sites measured by NMR and deduced from the XRD measurement show apparent discrepancies. It is, therefore, shown th at the choice of the space group has to be made in such a way that both dif fraction and NMR measurement can be brought to a consistent description of the sample considered. In the case of Al-30 delta -Taulelle, a new refineme nt of diffraction results has been performed changing the space group from C2/c to Cc leading to consistent results between XRD and NMR. (C) 2001 Else vier Science Ltd. All rights reserved.