Electric field gradient tensors at the aluminum sites in the Al2SiO5 polymorphs from CCD high-resolution X-ray diffraction data: Comparison with Al-27 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 de rive the electron density distribution in three Al2SiO5 polymorphs: andalus ite, sillimanite, and kyanite. The Al-27 nuclear quadrupole coupling tensor s 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 es timations of the Al-27 Sternheimer shielding factor, R and the antishieldin g factor, gamma, a close agreement was achieved between the quadrupole coup ling tensors at the various Al-sites derived from the X-ray diffraction dat a for all three polymorphs of Al2SiO5 with those determined previously from single-crystal Al-27 NMR spectra.