EFFECT OF PARTICLE-SIZE ON THE ROOM-TEMPERATURE CRYSTAL-STRUCTURE OF BARIUM-TITANATE

The room-temperature tetragonal-to-cubic transformation in BaTiO3 powders with decreasing particle size has been carefully studied, using materials prepared mainly by hydrothermal methods. Hydrothermal BaTiO3 powders exhibited a more uniform particle size distribution than oxalate-route powders, with x-ray diffraction and electron microscopy indicating that powders less than or equal to 0.19 mu m in size were fully cubic while powders greater than or equal to 0.27 mu m were completely tetragonal (within a 5% detection limit for cubic material) at room temperature. The tetragonal-to-cubic transformation temperature was also found to lie int he range of 121 degrees +/- 3 degrees C for BaTiO3 powders with room-temperature (c/a) values > 1.008. No transformation could be detected using differential scanning calorimetry for BaTiO3 particles with a (c/a) < 1.008 at room temperature. BaTiO3 powder with a particle size just too small (0.19 mu m) to be tetragonal at room temperature remained cubic down to 80 K. Different models for the cubic-to-tetragonal room-temperature transformation are discussed. Hydroxyl ions do not appear to greatly affect the cubic-to-tetragonal transformation, which appears to be essentially dependent on particle size. It is concluded that a model based on surface free energy, as previously discussed for the monoclinic-to-tetragonal transformation at room temperature of fine ZrO2 particles, is consistent with the experimental data.