While point-multiple models provide approximate agreement with observed field gradients in alkali-halide solid solutions, there are still quantitative discrepancies. In an attempt to understand the source of disagreement, we have analyzed the influence of the finite extent of the ions, taking into account contributions due to the electronic orbitals both on the solvent nuclei where the field gradients are measured and on adjacent ions. These electronic contributions are comparable to that from the point-multipole model for ions neighboring the solute and are, in the proper direction to improve agreement with experiment for the positive-ion nuclei, but their inclusion worsens agreement with experiment for negative-ion nuclei. Possible implications of these results concerning the overlapping-ion model for the solid solutions are discussed.

• Received 12 August 1968

DOI:https://doi.org/10.1103/PhysRev.184.989