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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

¹ Crystallography Group, Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, England
² Laboratoire de Cristallographie et Modélisation des Matériaux Minéraux et Biologiques, LCM³B, UPRES A CNRS 7036, Université Henri Poincaré, Nancy 1, Faculté des Sciences, Boulevard des Aiguillettes, BP 239, 54506 Vandoeuvrelès-Nancy Cedex, France
³ Mineral Physics Group, Department of Geological Sciences, Box 351310, University of Washington, Seattle, Washington 98195, U.S.A.

Correspondence: ^* E-mail: ghose{at}u.washington.edu

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 ²⁷Al 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 ²⁷Al Sternheimer shielding factor, R and the antishielding factor, , 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 ²⁷Al NMR spectra.

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