Solid solution hardening has been reviewed with respect to existing experimental data. It is shown with the aid of new experimental data on two inorganic systems (NiO-CoO and CaF2-SrF2) that the large increase in hardness of intermediate compositions correlates qualitatively and can also be quantitatively justified by the use of selected values of the parameters in a previously developed atomistic hardness formula. The key parameters are the variations in the unit cell dimensions, the eigenfrequency, and anharmonic factor of the structure.
J. N. Plendl, P. J. Gielisse, L. C. Mansur, S. S. Mitra, A. Smakula, and P. C. Tarte, "Interpretation of Solid Solution Hardening with Vibrational Spectra," Appl. Opt. 10, 1129-1133 (1971)