The electric field-gradient tensor has been computed using the point multipole model at the two inequivalent Al sites and at the Be site in chrysoberyl crystal. The calculated asymmetry parameters, the orientation of the principal field-gradient axes, and the quadrupole coupling constants are compared with values obtained from NMR measurements on the single crystal. The oxygen-dipole contributions to the field gradients are found to be very sensitive to the assumed oxygen polarizability. As the exact value is not known, calculations are carried out for three values: 2.19, 1.34, and 1.0 ${\mathrm{\AA }}^3$, the former two have been used in earlier investigations on ${\mathrm{Al}}_2$${\mathrm{O}}_3$ and BeO. The results for the three polarizabilities are discussed in the light of experimental observations. The deviations between calculated and observed values of the quadrupole-interaction parameters appear to decrease as the oxygen polarizability is varied from 2.19 to 1.0 ${\mathrm{\AA }}^3$, and best over-all agreement is obtained for $\alpha =1.0\mathrm{}$ ${\mathrm{\AA }}^3$.

• Received 30 September 1966

DOI:https://doi.org/10.1103/PhysRev.160.274