Effect of water density on polymorph of BaTiO3 nanoparticles synthesized under sub and supercritical water conditions

The effects of water density on polymorph of BaTiO3 particles synthesized hydrothermally under sub and supercritical conditions have been studied. Experiments were performed within the temperature range of 300 to 420 °C and the pressure of 20 to 40 MPa using a flow reaction system. These conditions correspond to the water density of 0.15 to 0.70 g/cm3. Aqueous TiO2 sols and Ba(OH)2 solution were used as starting materials. Characterization of products was performed by powder X-ray diffraction, transmission electron microscopy and Raman spectroscopy. From the results, it has been found that the crystal phase of products depends not only on the temperature but also the pressure. The BaTiO3 particles with tetragonal phase were obtained under the supercritical conditions, when the density of water was smaller than 0.5 g/cm3.