Redetermination of Nuclear Quadrupole Moments from Hyperfine Interactions in Compounds

A general expression is derived for the electric field gradient resulting from the overlap distortion of cation $p$ orbitals by the surrounding ligands. This contribution can be larger than that arising from point charges and electric dipoles in the lattice. The inclusion of overlap distortion in the determination of the nuclear quadrupole moment $Q$ of ${\mathrm{Al}}^{27}$ in ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ reduces $Q$ from 0.377 to 0.155 b. The latter value is in good agreement with 0.149 b obtained from atomic-beam experiments.