Edge- and screw-dislocation velocities in ``pure'''' MgO single crystals have been measured as a function of stress, temperature, and valence state of the iron impurities in order to identify the rate-controlling drag mechanism for dislocation mobility. Edge dislocations have been observed to move faster than screw dislocations in both the valence states over the stress and the temperature regimes investigated. Both the edge and screw dislocations move faster in reduced (Fe+2) samples than in oxidized (Fe+3) samples. From the analysis of the edge- and screw-dislocation velocity data in terms of the activation parameters (activation volume, activation enthalpy, total activation enthalpy, and the stress exponent of dislocation velocity) it is suggested that the edge- and screw-dislocation mobilities in ``pure'''' MgO single crystals in the reduced state are controlled by Peierls mechanism with thermally activated double-kink nucleation as the rate limiting step. The total activation barriers for edge- and screw-dislocation mobility has been found to be 11 and 17 kcal/mole, respectively. The calculated values of the Peierls stress for edge- and screw-dislocation mobilities in ``pure'''' magnesium-oxide crystals in the reduced state are 0.6×108 and 1.7×108 N m-2, respectively. In oxidized MgO crystals the edge- and screw-dislocation mobilities are suggested to be governed by a mixed mode consisting of Peierls stress and the resistance due to nonsymmetric defects (FeMg.-VMg″).
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