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The nanocrystal-formation mechanisms of barium titanate (BaTiO3) particles in a hydrothermal refluxing process have been studied. The refluxing method allows in situ sampling and requires ambient conditions (i.e., temperature below the boiling point of solvent and atmospheric pressure). It was found that the formation of BaTiO3 from the amorphous precursor was a very rapid process. BaTiO3 nuclei were formed at around 75 °C, 15 min after the beginning of the hydrothermal reaction. Individual BaTiO3 particles with dimensions in the range of 70–130 nm were formed 5 min after the nucleation of nanocrystals. Further reaction for crystal growth proceeded by consumption of the remaining precursors and possible aggregation of BaTiO3 particles via Ostwald ripening. It is proposed on the basis of the microscopic observations that the in situ transformation mechanism, rather than dissolution–precipitation mechanism, is dominant in the early stage of BaTiO3 formation.
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