Two kinds of twinning morphologies of ZnO crystallites prepared under hydrothermal conditions were obtained. The twin relations of the crystallites could be influenced by additives. The twinned crystallites in the pure H2O or weak basic solutions (1 N KOH) are bipyramidal and take (0001‐) as the twin and composition plane, whereas the twinning morphology of the crystallites obtained from 4 N KBr or 3 N NaNO2 solutions is dumbbell‐like and takes (0001) as the composition plane. Various twinning mechanisms based on the linkage of the growth units are suggested. The formation of twin morphologies of ZnO crystallites clearly demonstrate that twinning of crystals is either a result of differences between symmetrical and energetic most favourable structure arrangements or due to the consequence of oriented intergrowths.
The morphological features of ZnO crystallites influenced by solution basicity under hydrothermal conditions have been studied from the standpoint of the incorporation of the growth units. A crystal chemistry approach is developed to understand and thus to control a desirable morphology of the crystallites. The effects of the additive OHon the crystal morphology of ZnO crystallites are qualitatively but satisfactorily explained by a mechanism considering two aspects: (1) solution structures and the structural forms of growth units under a certain growth condition, i.e. the interactions between the solvent molecules and the growth units; and (2) the influence of the solvent molecules or additives on growth interfaces, particularly on two polar faces of ZnO crystallites in terms of the inhibition or promotion of the incorporation of various growth units depending on the solution basicity. Since the incorporation rates of the growth units are different on positive and negative polar faces, the relative growth rates of these faces are different and thus lead to the habit modifications. The approach clearly demonstrates that the hindrance of crystal growth is a consequence of surface adsorption processes.
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The twinning morphology of β‐BaB2O4 crystal grown by TSSG has been studied on the basis of crystallography. It was found that the faces R{101‐2} and r{011‐4} are often intergrown together by either r{011‐4} intergrown in R{101‐2} or in reverse, two individuals mostly take X(112‐0) and occasionally take Y(011‐0) as twin boundary; and that two individuals in the twin could take the same or opposite crystallographic polarity, this was identified by surface etched figures and the piezoelectric measurements. The formation of the twinned crystals are discussed from the crystallographic structural characteristics and the energetic considerations. The experimental factors such as crystal seed quality are suggested to reduce or eliminate the twin formation and to improve the crystal quality.
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