Electric field gradient tensors at the aluminum sites in the Al2SiO5 polymorphs from CCD high-resolution X-ray diffraction data:: Comparison with 27Al NMR results

High-resolution single-crystal X-ray diffraction intensities recorded with a charge-coupled device detector at low temperature (100 K) were used to derive the electron density distribution in three Al2SiO5 polymorphs: andalusite, sillimanite, and kyanite. The Al-27 nuclear quadrupole coupling tensors were estimated from both the internal polarization contribution of the Al electron density and that of the lattice with electronic multipoles up to octupoles for oxygen atoms and hexadecapoles for Si and Al. Based on new estimations of the Al-27 Sternheimer shielding factor, R and the antishielding factor, gamma, a close agreement was achieved between the quadrupole coupling tensors at the various Al-sites derived from the X-ray diffraction data for all three polymorphs of Al2SiO5 with those determined previously from single-crystal Al-27 NMR spectra.