Effect of hydrothermal temperature on the tetragonality of BaTiO3 nanoparticles and in-situ Raman spectroscopy under tetragonal–cubic transformation

Highly crystalline BaTiO3 particles were hydrothermally synthesized at 250–400°C from the reactants of TiO2 sol and Ba(OH)2 solution. X-ray diffraction measurement patterns revealed that the crystal phase of the obtained particles was tetragonal, whereas tetragonality (c/a) was difficult to estimate due to the peak broadening of BaTiO3 nanoparticles. The particle sizes determined based on the BET surface area and crystallite sizes were almost constant at 80 and 40 nm, irrespective of the hydrothermal temperature. Based on TG–DTA analyses, the lattice OH content decreased with increases in the hydrothermal temperature. In-situ Raman spectra were conducted in the temperature range from 25 to 200°C to follow the tetragonal-to-cubic transformation. The tetragonal-to-cubic phase transition was clearly observed at 120–130°C for the BaTiO3 hydrothermally synthesized at 400°C, whereas the BaTiO3 hydrothermally synthesized at 250–350°C exhibited a delayed transition at above 150°C due to peudo-cubic behavior. The tetragonality of BaTiO3 synthesized at 400°C can be assumed to be prominent owing to the lower number of lattice OH groups.