Self-Consistent Ionic Potentials, Fields, and Field Gradients at the Lattice Sites of Corundum (α-Al2O3)

Abstract

The electrostatic potentials, fields, and field gradients at the lattice sites of corundum (α‐Al₂O₃) have been computed self‐consistently, taking into account the ionic dipole and quadrupole moments of the oxygen ions. The dipolar and quadrupolar polarizabilities were used as parameters to fit the nuclear quadrupolar coupling data of ²⁷Al. It is found that the induced quadrupoles make a substantial contribution. The ionic polarizabilities needed for the fit are within reason. The electrostatic field and field gradient at the aluminum sites are predominantly described by a point charge and a quadrupolar lattice, the most probable oxygen quadrupolar polarizability being $\text{α\hspace{0.17em}=\hspace{0.17em}0.2–0.3}\mathrm{\AA }$ . The contribution of the dipolar lattice to the field gradient at the aluminum sites is small due to the accidentally nearly vanishing field at the oxygen positions. Consequently, the field gradient at the aluminum sites is insensitive to the O²⁻ dipolar polarizability. As the potential distribution appears to be almost entirely determined by the particular electronic density distribution of the oxygen ions and as the ionic polarizabilities of O²⁻ are not accurately known, an estimate of the covalent participation in the predominantly ionic bonds of Al₂O₃ is not possible. The nuclear quadrupole coupling tensor of ¹⁷O is estimated by use of the computed field gradient at the oxygen sites.