Effect of Particle Size on the Room‐Temperature Crystal Structure of Barium Titanate

The room‐temperature tetragonal‐to‐cubic transformation in BaTiO 3 powders with decreasing particle size has been carefully studied, using materials prepared mainly by hydrothermal methods. Hydrothermal BaTiO 3 powders exhibited a more uniform particle size distribution than oxalate‐route powders, with X‐ray diffraction and electron microscopy indicating that powders 0.19 μm in size were fully cubic while powders 0.27 μ 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 in the range of 121°± 3°C for BaTiO 3 powders with room‐temperature ( c/a ) values > 1.008. No transformation could be detected using differential scanning calorimetry for BaTiO 3 particles with a ( c/a ) > 1.008 at room temperature. BaTiO 3 powder with a particle size just too small (0.19 μm) 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 ZrO 2 particles, is consistent with the experimental data.