A hydrothermal method was used to epitaxially grow 70-nm-thick tetragonal (Bi,K)TiO3 films with stoichiometric compositions at 240 °C on (001)cSrRuO3//(001)SrTiO3 substrates. Crystal structure analysis revealed that the obtained (Bi,K)TiO3 films had a polar-axis orientation attributable to the matching of the in-plane lattice with the SrRuO3 underlayer. Large coherent displacement of A-site ions along the polar direction observed by transmission electron microscopy may have induced the large tetragonal distortion of c/a = 1.046. The dielectric constant was about 100 and was almost frequency independent in the range of 103–105 Hz, while the dielectric loss, tan δ, was below 5%. Well-saturated ferroelectric polarization−electric field (P−E) hysteresis loops were observed, and the remanent polarization (Pr) was 84 μC/cm2. In addition, the effective piezoelectric constant, d33,eff., was estimated to be 85 pm/V by time-resolved x-ray diffraction measured under an applied electric field. The Pr and d33,eff. values were larger than those reported for polar-axis-oriented epitaxial tetragonal Pb(Zr0.35Ti0.65)O3 films, indicating that the polar-axis-oriented tetragonal (Bi,K)TiO3 film has high potential as a lead-free material for various applications. Specifically, the small εr and large d33,eff. led to an improvement in the figures of merit [d33,eff./εr and (d33,eff.)2/εr], the most important index in sensor and energy-harvester applications.
Tetragonal (00l)-oriented epitaxial (Bi,K)TiO3 films were grown at 240 °C on (100) cSrRuO3//(100)SrTiO3 substrates by the hydrothermal method. KOH aqueous solutions and Bi(NO3)3 · 5H2O and TiO2 powders were used as the starting materials. Film thickness was controlled from 33 to 1200 nm by changing the deposition time, and the Bi/(Bi+K) ratio in the A-site of perovskite ABO3 was almost constant for all film thicknesses. Polar-axis (00l)-oriented epitaxial (Bi,K)TiO3 films were obtained without a secondary phase and/or other orientation for all thickness ranges. Large ferroelectricity with the remanent polarization ( Pr) of about 84 µC/cm2, comparable to previously reported lead-based ferroelectric films, was observed for (Bi,K)TiO3 films down to 33 nm in thickness. On the other hand, Ec increased with decreasing film thickness, but did not show strong film thickness dependence like other perovskite ferroelectric films. These data are very useful for understanding the degradation mechanism of ferroelectric thin films.
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