Low-temperature deformation and dislocation mobility in pure and Mg-doped LiF crystals

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Abstract
Two batches of LiF crystals, one essentially pure, and the other doped with 140 p.p.m. of Mg, were cooled from high temperature at different rates. The mechanical properties of these crystals were investigated in the temperature range 77 to 360 K. The results obtained are analysed and discussed in terms of the theory of interaction between dislocations and tetragonal defects for which improved expressions are derived. Good agreement is obtained between the predictions of the theory and experimental results. It is concluded that the thermally-activated deformation and low-temperature dislocation mobility are controlled by the interaction between dislocations and Mg2+ vacancy dipoles. The largest concentration of isolated Mg2+-vacancy dipoles is retained in the doped fast-cooled crystals which have the largest yield stress at low temperatures. It is estimated that in the doped slowly-cooled crystals one sixth of the total number of Mg2+ ions aggregate into complexes (containing more than 10 ions) which are responsible for a large increase in the athermal component of the yield stress. The values of m∗ in the relation V = A(τ∗)m∗ are independent of impurity concentration and are a function of temperature only. Results obtained by other workers are shown to be consistent with the theory and with these conclusions.