Electron Microscope Observation of Precipitates on Grown-In Dislocations in MgO

The defect structure of undeformed, single-crystal, Norton MgO has been examined by transmission in the electron microscope. It is observed that precipitate particles in the form of 0.2-μ diam spheres and short rods are present in large numbers on as-grown dislocations. By using special etching and optical techniques which are described, the concentration of the precipitate spheres is shown to average ${10}^9$ "balls"/cc (but range from ${10}^7$ to ${10}^{10}$ "balls"/cc) for a large number of samples examined. Qualitative evidence is presented which indicates that these precipitates are the origin of Tyndall scattering frequently observed in MgO.