On the surface morphology of solution annealed Co1−xO–MgO—Effects of directional dislocation exposure and Co1−xO condensation

The sintered Co1−xO ceramics with or without 20 mol% MgO solid solution in the rock salt type structure were annealed in the temperature range of 400–1500 °C in air for surface morphology development study. Electron microscopic observations indicated the MgO component considerably suppressed the thermal etching and the nucleation of the Co-rich spinel as expected. Surprisingly, prolonged annealing at 1500 °C caused anisotropic development of the {1 1 1}/{1 0 0}-faceted etch pits/hillocks from the cubic crystal system, which can be rationalized by the predominant exposure of 〈1 1 0〉 oriented dislocations on the {1 1 1} surfaces. Meanwhile, sublimation–condensation at this temperature caused cube-like Co1−xO crystallites to deposit preferentially on the (1 0 0) surface following parallel or 45° off crystallographic relationship via Brownian motion of the crystallites.