The hardness of alkali halide crystals containing divalent ion impurities

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Abstract
It was shown 30 years ago that the hardness of alkali halides increases considerably when low concentrations of divalent cations are incorporated in the crystal lattice. Recently, dielectric measurements have provided information about the manner in which the divalent cations are incorporated in the lattice. The measurements reported in this paper on NaCl: Ca++, NaCl: Mn++, KCl: Sr++, KCl: Ba++ and LiF: Mg++ were undertaken to observe the effect of the state of aggregation of the divalent impurities on the critical resolved shear stress. The principal results are: (1) the increases in critical shear stress is proportional to c2/3, where c is the concentration of divalent ion-vacancy pairs, (2) there is no increase in hardness as these divalent ion-vacancy pairs aggregate into groups of three (trimers), (3) in NaCl: Mn++, KCl: Sr++ and KCl: Ba++ there is no increase in hardness as these trimers grow into larger aggregates, (4) in LiF: Mg++ there is a large increase in hardness as the trimers grow into larger aggregates and (5) in NaCl: Ca++ the hardness increases as a second region of dielectric absorption appears. It is concluded that although the structure of the trimer is the same in all these crystals, the trimer can grow in two ways, one of which produces an increase in the resistance to movement of dislocations. It is suggested that the aggregates which contribute to the increase in hardness in NaCl: Ca++ are different in structure and do not grow from the trimers.