Crystal growth of cubic BaTiO 3 in the presence of polyethylene is investigated step by step using powder X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Titanium precursor Ti(OC 4 H 9 ) 4 aggregates with PEG to form spherical colloidal particles at the very beginning. Multiple nucleation of BaTiO 3 takes place on the surface of these colloidal particles. The nanocrystallites then selfadjust their orientations likely under dipole−dipole interaction and/or intercrystallite interactions enhanced by surface adsorbed polymers, followed by an orientated connection and crystal extension via an Ostwald ripening process. The final BaTiO 3 crystals have a novel dodecahedral morphology. The formation mechanism is proposed to be attributed to the selective adsorption of PEG molecules on the {110} crystal planes, significantly reducing the crystal growth rate on these surfaces. A kinetic model is proposed based on the calculated crystallite sizes using the Scherrer equation. The physical meaning of the model and a significant fake reduction of the crystallite size is discussed. ■ INTRODUCTIONNucleation and early stage crystal growth are of crucial importance in relation to crystal engineering and synthesis of new materials. According to the classical theory of crystal growth, nucleation in a hydrothermal system is normally associated with a supersaturation phenomenon and is initiated by the aggregation of sub-nanosized chemical species, for example, ions and molecules (monomers). After a nucleus reaches a critical size, the growth typically takes place by further attachment of monomers to its surface, and the final morphology of a free crystal in a synthetic solution is dominated by the slow-growing faces because the fast-growing faces may grow out and not be represented in the final crystal habit. 1 In contrast, the achievement in the growth of various nanostructured materials in the past decade has suggested several nonclassical pathways where crystals may develop from an initial mesoscale solid species. 2−7 For example, a faceted nanocrystal could evolve from an amorphous colloidal particle by nucleation beginning at its core and extending to its edge or vice versa. During the incubation period, one single crystal is developed from each colloidal particle (Scheme 1a). 8 Surface crystallization of a disordered aggregate may lead to a reversed crystal growth route (Scheme 1b). 9−11 The formation of crystalline shells with a faceted polyhedral morphology indicates that Curie and Wulff's theory 12,13 is more general than the Bravais−Friedel−Donnay−Harker (BFDH) law. 14−16 The latter produces a kinetic measure of the crystal growth rates along different crystallographic orientations. The former gives a thermodynamic view in order to predict that the equilibrium shape of a free crystal is the shape that minimizes its surface free energy. These assumptions can explain the formation of polyhedral habits in both the classic crystal growth route and the reversed crystal growth route.When mult...