Dislocation Damping in Magnesium Oxide Crystals at Low Frequencies

Amplitude dependent internal friction measurements have been made on MgO crystals over the temperature range 0° to 300°C at frequencies of ∼5 cps. Analysis of the temperature dependency of the critical breakaway strain, delineating the transition from amplitude independent to amplitude dependent internal friction, permits an evaluation of the relative pinning strengths of the Fe2+ and Fe3+ ions, determined on specimens from bulk sources. The Fe3+ ion exhibits a binding energy of 0.035 eV, whereas the Fe2+ ion has either a lower binding energy and/or a lower mobility which precludes the development of an equilibrium atmosphere. Optical absorption measurements confirm the presence of the Fe3+ ion in one bulk crystal and its absence in the other.