Substrate-suppressed phase transition in nano-crystalline BaTiO₃ thin films

Upon substrate removal nano-crystalline cubic BaTiO₃ thin films transformed into the tetragonal phase. Randomly oriented nano-crystalline BaTiO₃ films were prepared with moderate residual tensile stress by sol-gel processing or RF sputtering. The low dielectric constant (110 ± 20) and the X-ray diffraction spectra identified the films as being cubic at room temperature. Despite the tensile stress in the substrate-supported films, the free-standing films became corrugated, indicating 0.3-0.5% lateral expansion. The fact that the cubic-to-tetragonal phase transition was responsible for the lateral expansion has been confirmed by a fivefold increase of the dielectric constant (620 ± 10) and a detectable piezoelectric effect.