Effect of thermal strain on domain fraction in a-/b-axis-oriented epitaxial Bi₄Ti₃O₁₂ films

a-/b-axis-oriented epitaxial Bi₄Ti₃O₁₂ and neodymiura-substituted Bi₄Ti₃O₁₂ films with a different a-domain fraction, V₍₁₀₀₎/[V₍₁₀₀₎+V( ₀₁₀)], were grown by metalorganic chemical vapor deposition above the phase transition temperature. It was demonstrated that the saturation polarization observed for the a-/b-axis-oriented film is proportional to the a-domain fraction estimated by x-ray diffraction. The liner relationship passing through the origin revealed that the 90° domain switching by an external electric field hardly occurred. The extrapolation gave spontaneous polarization of 58 μC/cm for a pure a-axis-oriented (Bi_3.5Nd _0.5)Ti₃O₁₂ film. The domain fraction was investigated as a function of thermal strain originated from a difference in thermal expansion coefficient between the film and substrates. The domain fraction of the films changed with the thermal strain along the in-plane [010] in tetragonal a-axis-oriented films as well as epitaxially grown tetragonal Pb(Zr,Ti)O₃ films.