Morphology of MgO smoke crystallites upon etching in wet environment

The etching of MgO smoke in wet conditions – an archetype for aqueous dissolution of oxide crystals – is explored on the basis of transmission electron microscope (TEM) analyses. In pure water with neutral pH, in acid solutions and in moist air, the MgO cubic crystallites show first (1 1 0) truncations of the 〈1 1 0〉 edges and then (1 1 1) cuts at the corners. Criteria are given to characterize (1 1 0) and (1 1 1) cuts on the TEM images. It is shown that (1 1 0) truncations appear independently of the formation of the (1 1 1) cuts and are associated with a stationary state. The picture is fully consistent with the previous suggestion of a “constrained Wulff profile” [9] which explains the formation of (1 1 0) cuts by excluding (1 1 1) facets.