SYNTHESIS, CHARACTERIZATION, AND SELF-CONTROLLED ORTHORHOMBIC TO TETRAGONAL POLYMORPHIC TRANSFORMATION IN BaTiO3 NANOPARTICLES

The phase formation and thermal-induced phase transformation are studied in BaTiO 3 nanoparticles. 2 h of heating a polymer precursor at 550°C in air formed a single phase BaTiO 3 of 15 nm average crystallite size D. The X-ray diffraction peaks are analyzed assuming a P nma orthorhombic (o) crystal structure of lattice parameters a = 0.6435 nm , b = 0.5306 nm , and c = 0.8854 nm . The lattice volume V = 0.3023 nm 3, with z = 4 formula units, yields a density ρ = 5.124 g/cm 3. This is a new polymorph in comparison to well-known P m3m tetragonal (t) structure, V = 0.0644 nm 3 or ρ = 6.016 g/cm 3 (z = 1). An o ↦ t transformation appears on heating at temperature as high as 650°C in air. A proposed model explains the transformation above a certain D value in terms of the Gibbs free energy. Unless heating above 750°C, the two phases coexist in a composite structure (D≤27 nm ), with as much residual o-phase trace as ~28 vol%. As a function of temperature both the phases decrease in the V values up to 0.2975 and 0.0643 nm3 at 750°C respectively (0.0650 nm3 at 650°C). This is an important parameter for designing useful ferroelectric and other properties in a hybrid composite structure.